OK, not finding much of this topic. My 1982 300D has recently developed a rattling sound when the transmission shifts.... The tranny has not shifted extremely smoothly since I bought the car back in November, but it has not been too bad. The tranny is still shifting the same way, no slipping, good power, etc. It kind of reminds me of an engine mount problem I had years ago on a 1973 Plymouth Fury III. When I hit the gas the engine would move causing the cooling fan to hit a plastic shroud around the fan. I destroyed that shroud before I figured that on out. I also wish it was summer time. Even in my garage, the temperature is below freezing. Makes working on the car much less fun.... Note to self, next time you buy a 30 year old car as your daily driver, but it in April or May....

The only experience I ever had with a rattle from that area
turned out to be the bracket that goes from trans to exhaust.

Two bolts hold the bracket to another bracket bolted to the trans.
Rubber insulates the two brackets from each other.

I could not move the parts by hand so assumed they were ok.
A few weeks later the rattle was bugging me so I took it all apart
and found that it was the problem, and although parts appeared tight, there was a broken bolt in there, worn through the side of the bolt.

Your problem could be different. That is just what happened to me on my W123 body car.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

I'd look at the exhaust mounts, the one at the trans and the rubber donuts in the rear. Also look at the trans mount, make sure it's bolted tightly and still in good shape. Look also at the flex discs on either end of the drive shaft, and make sure the bolts are tight. While under there look at the center bearing/mount. You should not see any cracks in the flex discs. And you should be able to move the driveshaft back and forth at the center bearing no more than about a 1/2" side to side. If you more play than that it needs replacing. Also if the center bearing looks torn at all, it's time for a new one.

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Try resting your hand on the gearshift lever while driving; see if this makes the rattle go away. If so, it's just the lack of plastic bushings in the gearshift mechanism. The bushings become brittle and fall out every 7 years or so. This repair can be put off indefinately. Well, as long as you can stand the rattle...