I'm at wits end with trying to subdue the interior noise/vibration from my 245's stock exhaust. I know 240s are noisy, but this is silly. I checked all the rubber mounts. I cut off the heat shields (the welds were broke). I even bolted a heavy bar from the M47's rear housing and welded it to the pipe, and welded a plate on the head pipes to tie them together. I installed new motor and tranny mounts. This all helped firm it up a good bit, but still the vibration, especially at lugging rpms. My next move might be replacement of the exhaust with one of the high flow types (IPD or similar). Anybody tell me if this will help - mind you that I'm not as concerned with noise levels going out the end of the tailpipe as I am about the vibration/booming noise in the cabin. Ideas?

Are you sure the baffles in the mufflers are good?? They do break up and vibrate. especially at deceleration
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'75 Jeep CJ5 345Hp ChevyPwr and two motorcycles: it wasn't Volvos safety , it was Longevity that sold me

I solved mine by tightning the stems that the rear muffler hangers attach too.
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1990 240- 230K- IPD cam, springs, bilstiens, 3''exhaust, euro lamps

I had this same problem too that drove me crazy for quite a while. However, I finally solved it:

At the point where the head pipes come together there is a small plate attached. This plate was causing the vibration. I placed a LARGE hose clamp around the pipes/plate and cinched it all up tight. Problem solved... at least for me.

Give that a try.