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MG MGB Technical - Rhythmic Drone sound

We have a pulsing low frequency sound audible starting about 2000 rpm and getting louder at about 3000 rpm. Going down hill foot off the throttle there is no sound. Cruse or going up hill with throttle on the sound is evident. Car has 75,000 miles and runs fine. On another site it was suggested to tighten the support strap from the exhaust pipe to the bell housing and that actually helped. Hood open and high idle the sound seems to be coming from engine bay. Would a worn/loose timing chain cause a rhythmic droning?
JRW Wiebe

"Hood open"

If it happens stopped and out of gear then it's not the main part of the gearbox or anything downstream of that, or wheel bearings. Any difference with the clutch fully disengaged?

If not then it can only be engine or exhaust. Timing chain/tensioner tends to clatter more than anything, and is visible by timing jitter on a strobe.

To check the exhaust use a length of hose with one end held to your ear, an move it from the manifold joints through the rest of the system including mounts..
paulh4

Several excellent points to look into. Thanks Paul!
JRW Wiebe

Engine and gearbox mountings---------??
William Revit

15 Years ago I replaced all the mount rubber bits but it's a possibility. The sound, while driving, feels like it's coming from forward. Vibration, as through a radio speaker, I expect could make a unibody car like the MGB one big low frequency speaker. An odd part of this sound is it's audible at cruise and throttle on, and quiet off the throttle.
JRW Wiebe

The engine and gearbox do move to different positions when accelerating as opposed to decelerating or cruising.

Could also be sundry pipes and cables rattling against the body at certain rpm harmonics.
paulh4

Can the vehicle be put on jack stands or a lift and still have the sound present? If so, it allows one to use the "bell" section of a stethoscope as a directional microphone pickup and identify exactly where the sound is coming from. If there is no sound, because the engine is not under load, that tells you something itself. From your description it might be that the sound only happens/is heard when the engine is under load. If so, motor mounts and transmission mounts should be inspected in case they are allowing some shifting of the engine, transmission, or drive line that is causing the problem.

Les
Les Bengtson

Have you removed the gear lever gaiter and changed the screws recently? A couple of years ago I mistakenly fitted the incorrect front screw which interfered with the gearbox remote housing.
It sounded like a drone aircraft over the car. Just a thought.
Craig M Harvey

It was suggested as a simple test to remove the fan belt checking both water pump and alternator. No sound so I replaced the alternator with a spare and the sound is gone. Thanks guys for your thoughts!
JRW Wiebe

This thread was discussed between 15/11/2021 and 17/11/2021

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