Ball joints make a nice bucking sound when they get worn out. There are some stabilizer bushings that hold the bar away from the body that could be worn out/gone allowing some sort of vibration. Can you hear it more in the back or the front? My 124 has a belly pan that makes a flapping noise over bumps, I dont remember my 123 coupe having one, but anything's possible.

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.