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MG Midget and Sprite Technical - mystery noise

My turn to ask for advice, in case this has happened to anyone else...
For years I have heard a clonk from the rear of my 1500 when going over bumps, which sounds just like a loose spanner lying in the boot, which in fact is what I attributed the noise to and tried to forget about it.
Recently, as UK owners will know, the roads here now are nearly as bad as cart tracks due to lack of maintenance and the said noise is now much more prevalent as potholes are hard to avoid.

So to prove to myself it was indeed something in the boot being naughty I cleared absolutely everything out, including the spare wheel, went for a run and ...yup, the noise was as bad as ever!

The rear of the car is all new or replaced, ie poly bushes, telescopic dampers, new springs , no rusty or loose panels and the axle has a reconditioned diff. and rebuilt bearings, so there is obviously nothing loose.

However I was wondering if the exhaust, or fuel tank, both replaced in the last 10 years, although they look fine could possibly have loose baffles and if so could that cause such a noise?

Has anyone had any experience of this sort of elusive noise and any suggestions to solve it?
JB Anderson

My rear end clonk is always the bolts in the rear LA dampers, they just need a slight tighten.

Clearly you have replaced them, but they only need the tiniest movement to give a stomach turning 'thud'.

Try and isolate the noise,find a pot hole that causes it and drive over the with one side, then the other.

Is it when your suspension comes up or goes down ?

Malc
Malc Gilliver

Possibly the tubular shocks are either bottoming out, or hitting the ends on full stretch? You can check either by experimentation or by calculation of the suspension movement as against the compressed and extended lengths of your shocks.

OR,
If its under braking, it could be the backplates moving relative to the axle if the 4 little backplate bolts are loose.
Guy W

Thanks both for your suggestions, shock absorbers are a possibility which I can check by temporarily disconnecting them and carefully driving over a bit of road that causes the noise.
I think the noise is not as heavy as a shocker bottoming would be, but it will be worth eliminating them as a cause.

My loose baffle theory doesn't seem to be getting any support!
JB Anderson

it'd be a heavy spanner to make a clonk?

did you check the very sides of the boot, the rear wing spaces

loose baffle, wouldn't you notice by revving the car when stationary

noise can travel from its source

just thoughts
Nigel Atkins

It is a hard noise to describe but it is a light clonk not a heavy thud, and I have checked down the sides of the rear wing recesses for stray tools.

I had thought it was the exhaust pipe as it passes under the axle coming in contact with the axle when going over a bump (it is that sort of noise) but I have checked and it doesn't seem to be that.

It is a bit of a mystery as I have been involved with building one-off cars for over 40 years and generally know what to look for, but this has me stumped!

I can live with it if I have to, I just hoped someone may have experienced exactly the same thing!

I replaced the tank about 12 years ago and it may have been one of these Oriental replacements, which made me think about a baffle inside coming adrift and making a clonk when going over bumps...daft idea?
JB Anderson

Could it be the exhaust hitting the handbrake cable assembly/rod or even the underside of the car when the diff moves, My 1500 does this but only under extreme travel i.e full extension downwards of the diff against the strap.
R W Bowers

Having had a tube shock bottom out, I can assure you it will be felt through the seat at least as much as heard from the rear of the car. Running out of shock extension though is another thing. Not sure what that would feel/sound like.

My B (tube shocks) has a similar (occasional) sound (LHS I think) and, like you, I chased that through possible exhaust contacts without finding any indication of contact.

I would think baffles would be more 'tinny' a sound.

RW may be close?

Frustrating, but it seems to have no effect on drivability so I have tended to accept it.
R Taylor

Way down the list but it's possible for a petrol sender float to tap the side of the tank.You can sometimes hear it by rocking the car back and forth to create a little wave.Anything non standard like lowering blocks?
F Pollock

Many thanks RW, RT and FP for your further very relevant suggestions.
I went out just now and jacked up the rear and the axle/handbrake mechanism is well clear of the exhaust and I think as you say it would have to be extreme travel before they would get close to contacting each other. This noise happens without extreme travel ocurring and it is not a jarring feeling/noise like a bottomed out shock absorber would give.

I tapped (quite firmly) the bottom of the tank with the flat of my hand and there is definitely a sound of something loose in there, the sender unit should move, obviously, but should not be "loose" surely.

If nothing else seems a likely culprit, I will take the tank out as a winter project and see if there is something amiss with it, although it is hardly worth buying a new one at their current prices.
That will keep me busy for a wee while!

Thanks again for your very helpful suggestions,as ever, much appreciated.
JB Anderson

In all this I've assumed were talking about a late model car say a 1500. Just crawled under mine sitting unloaded and took the attached pic, not much clearance say 50mm. My car is totally stock and pretty much everything underneath is new including the springs so it should be a good example of the expected clearance. The handbrake to diff rubber mount is vital in stopping rattles. HTH
Cheers
Rod

R W Bowers

I thought there was some thing wrong with that photo and then realised it was a reflection

Rod, sort that car out mate, get some road grit, dust muck on it






or you'll have to change those rusty wheels ! ;)
Nigel Atkins

Gday Nigel the're only slightly rusty on the inside, and of course polish works on the underside of cars also (TIC)LOL
R W Bowers

Thanks RW, I wish mine was as unmolested as yours!

I have much the same clearance, unloaded, as you but I will check again.

My handbrake outer cable is wire-tied to the bracket on the bottom of the diff, without any rubber mounting.It doesn't move so I presume it can't rattle!

Is that the mounting you referred to?

Thanks very much for the helpful picture, by the way.
JB Anderson

Rod, you and that car of yours need to get out more :)







or of course you could spend days bringing the inside of the wheels up to reflective polish standards :)

my car gets grit and dust even when it's just parked up, we keep getting sand blown in from some where, I'm thinking of collecting it and making a castle
Nigel Atkins

Another thought, I did replace the rear springs some years ago and it occurs to me that if one of the inner leaves had broken it might make a clonk type of noise, or do they stay silent until something nasty happens?
Anyone got experience of broken leaf springs?!
JB Anderson

Gday JB Yes this is the bracket which allows the cable some up and down movement. My bracket actually rests on the exhaust pipe when the car is jacked up, this being at the full extension allowed by the rebound strap. If you have telescopics fitted do you still have a rebound strap? without it the only thing stopping the diff is the shock, not really ideal.
Cut a piece of 5mm reinforced rubber to fix the bracket, see pic, I used 1mm steel and pop rivets to remanufacture (reinvent???) the bracket to my specs. I'll try to get a clearer pic if this doesn't help
Cheers
Rod PS Nigel yes it probably is too clean but I only go out if the sun shines, remember we get a lot more of this than you do so the car is really getting used.

R W Bowers

Hi JB Finally figured out how top resize my pics check this out

R W Bowers

Once again thanks RW!

I too have made my own version of that force reaction bracket which is very similar to yours and also just touches the exhaust when the axle is "hanging".

I had to make some replacement straps for my axle, as the originals were too perished, out of stainless steel cable and sleeved in rubber, so hopefully they are not guilty of noise making but who knows.

This morning I took off both telescopic shockers and bounced the car as hard as I could and there was not a single strange noise of any kind coming from the back..a bit of fuel sloshing, a bit of bush creaking and that was about it.

I think I am going to have to live with it, as I have done for a number of years already, and hope it doesn't get worse.I suppose it is just that I don't like mysteries!

Thanks again for your help.
JB Anderson

<< took off both telescopic shockers and bounced the car as hard as I could and there was not a single strange noise>>

So its the shocks? They do make a real harsh bang if they bottom out. But the noise is less "shocking" when on full extension so maybe that is it. Except that to get full extension you have to be going pretty fast over significant humps! Other possibility is if the shock eyes have large washers that are making contact?
Guy W

I see what you mean Guy.
I actually took the dampers off so I could get the body to move as much as possible relative to the axle by pushing down on the rear wings and getting a sort of rhythm going which the dampers would have prevented.
The car was bouncing quite a bit and there was no sign of handbrake linkage hitting exhaust for example.
It didn't occur to me that as there is no noise that it could have been the dampers causing it! What a clot! I don't think they are anywhere near full extension or compression when in use though.

The next thing will be to go for a drive, carefully, without the dampers and see if the noise is still there when going over a "known" bump.

Maybe just taking them off and putting them back on today may have changed something.
JB Anderson

OK JB, you now have me thinking. I wonder if the 'outer' (upper) shock case is contacting the 'inner' (lower) as the shock compresses?

Perhaps an issue of alignment of the shock between its upper and lower mountings. Or alignment moving 'off' during compression as the axle rises and/or moves laterally from its rest position?

You will probably get a look at yours before I will mine, so I would be interested in anything you may discover.

Regards

Roger
R Taylor

RT and Guy, I took off both dampers and went for quite an unpleasant run, wallowing all over the place and not wanting to corner convincingly even at low speed, and the noise was still there at my favourite bump.

So it is not the dampers or their fittings.

The noise actually sounds like two metal panels coming into brief contact with each other, like a tappy, flappy, clonk when going over the bump.
That is why I thought it was something loose in the boot, but it ain't that.The boot is completely empty.

There is absolutely nothing visible like a rusty panel half hanging off that would make such a noise so I am still baffled.

The car is going fine and handling as normal so I am not unduly concerned, and it recently passed its test so Mr Mechanic is happy there is nothing untoward lurking underneath.
Just a bit odd!
JB Anderson

One other thing to check is the boot lid stay bar (if it is the same as my B). Is it moving in its holding bracket? If so, try a short length of something like fuel hose (the clear mower type will do - just looking for a means of securing the bar without movement but enabling it to still slip in to the bracket.

Regards
Roger
R Taylor

how about the thing that holds the boot open?

or a spanner has got down inside the rear wing. that makes an annoying noise that takes ages to find...
Rob Armstrong

I have taken off the boot stay and checked down the recesses of both inner rear wings, but with no joy.

You can see how mystifying this noise is!

Thanks for the suggestions though.
JB Anderson

To add to the confusion....what about worn out boot lid rubber seals ?

Have you tried just crawling under with a box of wrenches and giving everything a twist that has a nut / bolt...that wod be the ezsist and fastest way to eliminate something loose... and zero in on anything not related to nuts and bolts

Prop
Prop and the Blackhole Midget

Have you looked inside the tranny tunnel?
Guy W

boot handle, lock mechanism, catch - loose ?
Nigel Atkins

I replaced the boot seal last week and I have checked and rechecked that everything is tight underneath.

The noise is from behind the seats, boot area, so the tunnel is not guilty I don't think.

It certainly sounds like something like the boot handle being loose but I have tweaked that too.

I will have another go!
JB Anderson

>>The noise is from behind the seats, boot area<<
goal posts moved a bit there

so what's in that area

brake pipe, fuel pipe, prop, handbrake cable - that me out of things
Nigel Atkins

Guy W, Cumbria, United Kingdom


Have you looked inside the tranny tunnel?


Hahahahaha

Gerbils, plastic ware, spark plugs....I can ONLY imagine what you could find in a "Trannys Tunnel""

Hahaha

Wow....its 4:00pm and the paint fumes have completely overtaken me today

Prop
Prop and the Blackhole Midget

My original post indicated I thought the problem may be in the boot area but also considered underneath components as well.

No goal posts moved anywhere.

If I knew where the flaming noise was coming from I would fix the darn thing.

Thanks for all those who have given helpful suggestions, it would appear we are now moving into the awkward area of trying to get help so perhaps we should just leave it there.

Another possibly helpful thread brought to its knees without warning.

Thanks.
JB Anderson

dear, oh dear, it's certainly got you rattled, I really meant the behind the seats bit but didn't want to quote out of context

I put the things I could think of at the time and I'm sure others will add to the those with better ideas

the helpfulness of the thread is still here and will no doubt be ongoing

lighten up a bit and stop being so prickly on this, calm down dear it's only a mystery noise and a few posts
Nigel Atkins

Ok the dynamics of the movement causing the rattle are forward motion and up/down, not a simple up down motion. Something might be moving fore and aft. HTH
Hang in there mate I for one am fascinated by this problem
Cheers
Rod
R W Bowers

I hate to imagine what Prop was thinking of when I suggested checking your Tranny Tunnel!
Guy W

Rod , I have taken the boot stay completely off, I had only undone one end before, and the lock mechanism seems absolutely tight.

Options are running out!

I won't be out in the car for a day or two but will report any further developements.

Like you, I am keen to find out what it is, but it does not stop me using the car.That is the main thing.

There were no Trannies in the tunnel luckily, they would have been very noisy.
JB Anderson

JB,

Maybe consider the possibility of loose boot lid hinge pins or loose boot lid stiffeners? Admittedly not likely, but you already tried all the likely ideas. You could take the boot lid off (including hinges) and test drive it.

Did you check for loose baffles in the exhaust, which you mentioned as a possibility. Maybe you can take off the rear and center hangers and then shake it vigorously to see if you hear anything.

Charley
C R Huff

Good thinking Charley about the hinge pins and stiffeners, I will have a check tomorrow.

I was leaving the exhaust (and tank) until I had tried everything else, in case I ruin something in the removal!

Looks like I may well have to check them out sooner rather than later if I don't find anything else.
JB Anderson

Perhaps my final contribution. JB's noise description so fits the case of mine. One other thing I have pondered (because the noise is not constant) is whether a heated exhaust system manages to expand by just enough to cause one of those very tight (but cold when investigated) component proximities to close just enough to be the cause.

A difficult thing to examine/estimate with the vehicle static.

Like JB, my noise does not appear to affect the cars performance or handling, so I just drive it on.

Regards
Roger
R Taylor

This thread was discussed between 14/08/2013 and 21/08/2013

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