The W124 has a bushing in the rear that commonly wears out and causes squeaks when going over uneven surfaces or bumps. And clunks when putting the car in gear.
It is the rear wheel carrier bushing, also known as the rear control arm bushing, and others have called it a ball joint (it's not) but it does have ball bearings in it.
Do a search and there should be some info.
I just replaced mine a few weeks back. About $50 for the 2 sides.
You need a special tool to remove and install the bushings. I did not need the spring compressor I rented from Brian, as a floor jack did the job easier.
I bought the bushing tool to use, and I put it in the rental program.
About 2.5 hours of work for both sides. Less if you've done it before.