I've got a an '86 300E with a vibration/rumble sound that occurs at speeds above 65 mph, and rapidly worsens with increasing speed. It seems to be coming from the rear of the car, and can be felt through the seats and 'in your chest'.
I've replaced the following so far:
- motor mounts
- transmission mount
- front and rear flex disks
- had drive shaft balanced with new center bearing and u-joint (vibration dampener was removed at this time)
- changed wheels and tires
- changed differential and transmission oil
The previous owner has replaced the rear differential mounts and rear subframe mounts, and one rear wheel bearing about 20k miles ago.
So far nothing as made a difference. The car has 220,000 miles on it, and has the 5 speed manual transmission.
The rear differential whines at low speeds, and there is a clunk from the rear end if I don't clutch smoothly.
What's the next most likely culprit?

Sounds like the differential is a gonner.

With the vehicle in neutral and the park brake set, check the pinion (where the driveshaft couples with the rear diff.) for any side to side and/or in and out slack. Slack will likely be accompanied by (much!)oil leaking from the pinion shaft seal.

If the pinion seems okay, I would suspect side bearings or C/V joints/shafts.

Rear diff. repairs are not complicated, but they are $$$$$, so good luck.

Hatch

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1980 240d 4sp
ASE Master Auto Technician
ASE Advanced Engine Performance

This sounds very familiar. I have an 300SL with 113,000 miles on the clock. It seems as if you have gone through the same process of replacing steering and suspension parts exhaustively with no results (but lots of expense).
My car often feels 'lumpy' in that it moves around on the road quite a bit. Diff does clunk and the are other squeaks and creaks from the rear at low speed. The vibration can be felt through the seat - not through the steering. I have recently replaced the pinion seal at the rear as there were quantities of oil being sprayed over the underside but I thought this was just a damaged seal rather than major diff problems. Oh dear!

Thanks for the advice. I'll try checking for play this evening. Is there any way to isolate whether the differential or CV joints/shafts are the problem? There is no oil leakage from the differential.

I've considered purchasing a used differential, is this a reasonable alternative or is it just as likely to have the same problem?

Thanks again.