Need help diagnosing a rattle

I have a 96 G 463 in the US. As long as I have owned the car (4 months) there has been a rattle in the rear of the car when going over a bump.
It is most noticeable when the right rear wheel hits a short dip in the pavement.

So far I have replaced the rear shocks, disassembled the right rear brake drum, checked for any looseness in the exhaust system including the crossover pipe.
I have removed the shield on the gas tank and checked for loose components. I have replaced the linkage arm of the ALB unit. I have attempted to tighten the bolts and nuts underneath the car but have found no loose or worn bushings.

So far nothing I, or a mechanic have done has helped to diagnose the rattle. It sounds similar as to when one tows a trailer and the coupling is moving on the hitch while travelling on rough roads.

Any help in diagnosing the fault would be greatly appreciated.

Pete