Thank you for the various recommendations. I called my technician and he initially thought it was the center driveshaft support and bearing, as suggested by both RAMBLIN and BRUNO_300TE; but after I told him the symptoms appeared only when the engine was fully warm, he thought that was very odd and backed away from this initial diagnosis.

(RAMBLIN, regarding the center support, you stated that you were "suprised the mechanic didn't notice [whether or not] it was bad..." when he swapped the transmission... I asked him, and he informed me that he never had the opportunity to get a look at it, since he simply pushes the entire shaft backwards and out the way. The mount was therefore never exposed for his inspection. And since I'd had absolutely no problems prior to the tranny-swap, he had no reason to check back there.)

Anyway, I returned the car to my technician and he tracked down the source of the problem easily enough: the new front flex disc was thumping up against the tunnel, which was what I could feel inside. He said that in over 25 years of working on Mercedes vehicles, he'd never before seen this occur.

Without actually determining why this was happening, he opted to simply treat the symptom. He marginally lowered the transmission by using two longer bolts and some spacers on the tranny mount assembly. By dropping the mount a fraction of an inch (and therefore the front end of the driveshaft), the disc can no longer physically make contact overhead and the horrible thumping is gone.

He said I should drive it for awhile and see how it goes. If I still notice problems, he wants me to bring it back and he'll check engine mounts and other possible root causes.

On the way home (about 25 miles), things were MUCH better. No thumping at all; however, when I hit the gas from a standstill, I can still note a minor vibration, so I know there remains instability, but it's certainly a far cry better than before.

One more thing: While my mechanic thought it a long shot, he did note that at the location where contact was being made--immediately above the front flex disc--resides the SRS electronics, and that an unanticipated triggering of the airbag(s) could have resulted from the shock.

Any additional thoughts or insights based on the new facts? Anyone ever seen something like this before??? Should I leave well-enough alone, or should I pursue perfection here?

(By the way, the center support was in great condition, so rule that out.)

__________________
Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)