Purchased a new to me '82 300D Turbo diesel with 99K miles. Somebody really loved this car. It's very much like new. However, soon after I purchased the car, I noticed a great deal of vibration coming through the steering wheel and some in the seat of the pants. Changed out the two front motor mounts. Passenger side was collapsed, despite looking "ok" from the fender. Driver side was mush as the primer pump leaked on it and destroyed the rubber. The rear transmission mount was changed. Oddly enough, the wrong transmission mount was previously installed. Someone put a non-turbo diesel mount on a turbo and stacked 3 washers on top of each other to take up the slack - it was pretty sad - it was the wrong mount for the car. The correct new mount sat beautifully in the crossmember like it should. I think the reason that the rear mount was changed, was that someone before me was hunting down the clunk in the rear of the car as it downshifts. The clunking is probably metal to metal now and really gets your attention, but almost exclusively as the car downshift. The rubber in the driveline support was simply decayed from time and that was replaced as well. I retained the driveline support bearing since they seem to good bad infrequently. I also changed out the original flex-discs, front and rear. WOW! Vibration nearly completely gone. Very smooth and probably the way the car should absolutely be from new.
The final item that seems to allude me is the clunking noise that I thought would have gone away with all the other rubber changes. As the car begins to slow down and downshift, a LOUD clunk sounds as the gear changes. It seems to be coming more from the rear and is only present on downshift. It's not coming from the transmission. If you think you might have an idea, I'd sure love to hear about it.
In the meantime, I'd sure recommend that 123 owners change their mounts, flex discs, and driveline support to get that really nice feel again. The car is a joy to drive!
Thanks!