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MG MGB Technical - Drumming in BGT.

No, I'm not a percussionist - I've completed a thorough tidying up of '73 BGT: bare-metal respray and (more than anticipated) rust repair; all new interior, including copious Dynamat on floor and walls; new Webasto (love it); new front springs, refurbished shockers, new steering linkages ... you get the picture. This is literally a daily driver, we live in a small country town where work is 5 minutes away.

The problem? - an annoying noise almost like a drumming, noticeable even at low speed before it can possibly be wind noise. It is evident with the Webasto open or closed; it improves slightly with both windows open; there is no apparent harmonic noise, just a low thrum in the ears.It sounds as though it's coming from under the floor but is difficult to place front or rear.

Has anyone here "been there, done that?" - I'm wondering whether a worn component of suspension rubber, gearbox mount, engine mount etc might produce this annoying noise? Obviously I could replace the whole kit & caboodle, but would love anyone's advice first!
Any advice gratefully received. Regards, John.

J P Hall

... and from the rear ... .

J P Hall

Hi John---looking good
Can you make the noise happen with the car stationary or is it speed related--If it's at speed, what speed is it at---------------
William Revit

Check the tyres for a stone, nail etc..
Allan Reeling

Willy, I'll do a test in the morning and clarify when the drum is noticeable.
Allan, will do - but it's a continuous drone, quite irritating to the ears until you block it out.
J P Hall

Hi John

What's the general condition of your tyres? A front to back swap might be worth a try.
Chris Woodfield

Did it do it before? Or is it new to you?

Take out carpet one section at a time, or add temporary padding, and see if it makes any difference.

Chris mentions tyres and they can cause some weird effects. I was convinced I had a bad wheel bearing on the ZS until I replaced the original tyres that were on the rear, and it stopped. Subsequently I read on one of the fora that it wasn't uncommon.

Update, and thanks for the suggestions so far:
No, I don't remember the noise before the car was laid up.
Tyres are good - Michelin XZX with about 2 years' wear.
The noise/drone is there from low speed onwards, but I now realize it's also there (but to a lesser degree) when stationary with engine running.

A DISCOVERY: after a test drive this morning, the drone seems to centre on the rear cargo area which is acting like a drum. On checking inside and underneath the cargo area floor, I notice that when I put the fuel tank back in, using new fixings, I omitted (forgot) to use the fibre/plastic spacers between the tank lip and the boot floor. So I'm thinking the noise might be transmitted from under the rear floor (eg muffler noise/vibration), starting as a fuel tank vibration which is able to resonate through the floor into the cabin.
I'll put spacers in between tank and floor during the week, and see if that makes a difference. Will report back in case it's of interest to BGT fixers.
Yours, John.
J P Hall

Did you also remember to put the foam/rubber strips on top of the tank?
Allan Reeling

IF you've had the exhaust off at all it might pay to check that the rear exhaust mounting is nice and free as in not bound up causing a resonance through the floor
William Revit

It might be a noise short. A physical connection between a moving part and the body. Perhaps across a foreign object such as a spanner. To make things more difficult sound can transmit away from its source to another place wherever it can resonate.
Sorry if this is "sucking eggs"
Roger Walker

Allan, yes - I carefully put non-water absorbing foam strips on the top; Willy, new bobbins on the rear exhaust hanger.
Roger - I hear what you're saying, and agree - I just haven't worked out where is the source of noise/vibration. I'm even going to look at the (new) tailgate seal, the one that fits in the channel on the gate, not the pinchweld that can be put on the body flange. Yours, John.
J P Hall

Years ago I fitted a stainless steel exhaust system which had a 'booming' resonance at some engine speeds. Various tailpipe trims didn't tune it out. Eventually we traced it to the muffler outer skin actually vibrating. A strapping strip wrapped round and tacked on cured it.

Other users or the manufacturer must have found the same, those systems started to be supplied with just such a banding.

It's often overlooked that the GT originally had a shaped piece of plastic foam fitted inside each C post to damp out sound resonances. These are often long gone, I've no idea how effective they are/were.

EDIT I don't know who Arial Small is ( ho, ho ) but it aint me,

This thread was discussed between 19/02/2017 and 21/02/2017

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