I've been having problems with a vibration that comes and goes in my '87 300D with 247Kmi. Its been going on for some time, but there have been overlapping problems with suspension, alignment, and tires which have helped mask the problem.
Its a vibration that I feel in the gas pedal, and steering, sometimes throughout the car. Its worse on decleration, going downhill than it is on level ground, and least noticable acclerating full throttle on an interstate entrance ramp. Its car speed related and seems to depened on driveline load. Changing from drive to third, or even neutral, will not affect the vibration (I've never changed it during max load, only level or downhill). The front end suspension is rebuilt, and most of the rear (except two diff bushings, subframe bushings, and inner control arm bushings). Shocks all around, front upper shock mounts, engine mounts, tranny mount are replaced. The alignment is in perfect order as of May and was verified to be perfect a few days ago. The car does not pull, the tires are wearing okay, and braking does not make the vibration worse.
At one point the vibration seemed like a bad half axle, but the dealer shop foreman was cautious and wanted to see the car. After asking some questions and going on a test drive, and checking out the driveshaft, he seemed to think the vibration was in the tranny. That was in May. Folks told me it would very unlikely.
Now its August, the problem is still with me. I think it may be getting worse, as it used to require a long trip and hot, sunny weather to be at its worst. Now, it seems to do it on shorter trips. I asked my mechanic to look at it again. He said it was the tranny, definitely not the driveshaft. He based this by holding the front driveshaft and turning the rear and not hearing any significant noise. But turning both together, there was a resounding "klunk" which he said is coming from the tranny. I've repeated this many times and know the klunk.
I'm just wondering if anyone has any input or feedback? How often does this happen? Other mechanic at the shop said it hardly happens. But with the dealer shop foreman, who has been spot-on with 2-3 problems AND my mechanic in agreement, I believe them.