Put simply, my rear end is on jack stands and I can see that both of the rear shocks have that oily residue, telling me they should be replaced. Since the car is jacked up with the wheels off, what else should I check? The knock comes right when I let off the accelerator (it's louder if the pedal is released suddenly or I was accelerating at a higher rate) and from the rear drivers side. It may be something that would happen at acceleration too, but my E300D is much better at slowing down. Right now I'm trying to compare left to right, but that isn't yeilding much. Brakes and flex disks are ok though.

Ha, based on your thread title, I was going to say..."well answer the door".

Check all the suspension bushings. It might be a good time to start replacing them all little by little. I am chasing similar issues and starting with the differential bushings and lower control arms on our '87 W124 300TD.

There is a thread to this effect about a W123 I believe. The process is the same, parts most likely different.

This can be tedious patience testing work, but the pay off in improved ride quality is worth it.

best of luck.

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83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.