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MG MGB Technical - Rear-end rattle

I've got a noise like a pebble being shaken in a tin can when pulling away. Its frequency increases with road speed and becomes inaudible past about 40mph. It's there in 1st/2nd/3rd, stops if you lift off and seems worse from cold.

I've run the car on stands and can't hear anything or see anything amiss other than slight play in the rear UJ. I can see nothing in the boot that would explain it.

I replaced the axle for a used but supposedly rebuilt one in December so am hoping it's not that, but feel free to tell me the worst!

Thanks,
Steve
SteveP

Check every single fastener at both shock absorbers (including the shock links) with a twist from a wrench (= spanner).
Daniel Wong

It could be your staarter bendix. sometimes they rattle like that you have described.
Sandy
SANDY SANDERS

What year? It might the needle bearings going out on the transmission layshaft since you mention that it is audible in 1st, 2nd and 3rd. RAY
rjm RAY

Thanks guys. It's a 74. Sounds like it's way back near the rear end, maybe on the LHS. I've shaken the exhaust to no avail but did wonder if a baffle could have come away. I'll check the shocks (Konis) tomorrow.
SteveP

Steve: Re-examine the UJ. The "slight" play you mention, may be just the culprit. Put the car up on jack stands again, put the car in neutral (engine off) and set the e-brake. Then get under and twist the drive shaft back and forth and look closely. It should be pretty tight. If you get movement with a clunk, you found it. UJ's generally make the noise especially under acceleration (under load)and the noise will trail off as you get up to speed since the load is greatly reduced.

Let us know how this works out

Cheers

Gary
79 mgb
gary hansen

Steve -
I agree with Gary. Since it is off the ground, disconnect the rear UJ and move it through its full range by hand - I can pretty well guarantee it will be crunchy! If not that one, then disconnect the DS at the gearbox and check the front one.

FRM
Fletcher R Millmore

Hi Steve,

This might be like telling Grandma how to suck eggs, but check that your wheels are on tight.

Herb
Herb Adler

I'll give you the worst. Worn thrust-washers in the back-axle also cause a pebblish noise. Of course they should have been replaced during overhaul, but how good was the overhaul?

Replacing them is doable (and fun) for the competent DIY'er but involves taking apart almost the whole backaxle, the parts involved are inexpensive.
Willem vd Veer

Diff thrust washers usually only cause a clonk when going from drive to overrun and back. The description and location sounds more like exhaust, but that would more likely be rev orientated and not gear or speed.
Paul Hunt 2010

Thanks everyone. I'm going to change out that UJ and review from there. I'll come back to you once I have news.
SteveP

Thanks for your help with this. I changed to UJ and all seems good. I love these cars; even I can fix them.
The UJ was much worse than it first appeared. It would lock into a "good" position with just a little play in it but in taking it off I could see it click out, presumably into the rattly position. It would explain why it was sometimes very noisy pulling away and other times fine. So there we have it: if it looks suspicious, suspect it!
SteveP

I've seen them get so bad that they destroyed the driveshaft. RAY
rjm RAY

This thread was discussed between 08/03/2010 and 26/03/2010

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