Things that go BUMP in the night

[dieseldaddy]

I have a strange thumping noise coming from the rear of my '81 300TD. I have not been able to find what it is, but here is my blind description:
Going over bumps sounds like something is bouncing up and down on my fuel tank. I can get the same results when car is parked and I bounce on the tailgate a little. Sounds metal...almost as if there were possibly an exhaust mount bracket or something loose and hanging. I REALLY do not want to have to remove the tank without asking the experts first!
Anyone had a similar annoying noise?

[DMarble]

I had a similar rattle that only ocuured when I would hit a sharp bump at low speeds. It turned out to be the foam blocks inside the rear bumper
(these are the impact absorbtion blocks) My car is a 124 so I don't know if the bumper on yours is constructed the same. It took me three years to isolate that clunk as it didn't happen all the time.