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vibration noise

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ceedub

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I seem to have developed a heck of a vibration noise in my '73 MGB when on compression in second gear coming down through 3000 rpm. It definitely sounds like a noise coming from metal vibrating on metal in the key of B flat ( I think ) with the sound appearing to be coming through the dash vents. It quickly disappears as the revs diminish. I have taken out the vents, the glove box and the radio and can't seem to find what it might be. I don't know if this is a vibration noise common to a 'B or if I just keep taking things off until I find it. Could it be the heater core? I took the fan motor off but no luck. Hope there is an answer.
 
Could be lots of things, but it's not common to "B's". Should not have vibration noises if all things are good. I'd start checking for exhaust leaks and then for loose exhaust hanger backets.
 
Only in second? That could be telling.

Is it actually just a noise or is it a vibration?

What kinda gearshift knob have you got?

At first blush, the description is one of a sympathetic vibration. I.E. a bunch of things all coming to a harmonic threshold at 3K with second engaged. If you hit th' loud pedal as it occurs, does it quit? If so, there's something in the gearbox goin' awry. Layshaft bearings come to mind on first blush.
 
It is a definite vibration like two pieces of metal vibrating together. The gearshift knob is the stock round black plastic with lock nut. The noise is only evident on compression and goes away on acceleration. The noise seems to be coming from under the dash but I have been fooled by sound before.
 
Could be a motor mount, allowing metal to touch metal. Block of wood on your floorjack, up against the sump, should tell.
Exhaust touching on deceleration engine torque.
U-joint.
Air cleaner hitting something on deceleration.........
 
As noise has a funny way of sometimes showing up, far from it's source, have you checked the plastic cup on the bottom of the gearshift lever? If that's cracked, broken, or missing, the lever will rattle at different RPMs. PJ
 
The only other thing I can think of to add to this list of 'possibles' making that kind of harmonic vibe is the fresh air flap. Change its position and check the gasket seal on the edge of the thing.
 
Thanks guys for your input. This forum has been such a tremendous help through the years. I really appreciate the knowledge base that I can draw upon when problems arise. Looks like I have a few things to check this weekend. Hope it is not the trans. I should have thought about the muffler angle. I had a '75 'B that had a similar but louder vibration which sounded like the diff. groaning. It actually was a loose baffle in the muffler.
Cheers from sunny Victoria.
 
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