How tight did you tighten the nuts holding the upper mount down?

They should only be far enough down the shaft for a single thread to show over the top nut if you have two, and only an equivalent amount if you have a nylock nut. Much more, and all the resilience in the rubber is used up by the nut compressing the buffers, so you get a solid thump transmitted.

Very common mistake, I didn't know about it when I did the 220D, on top of using Gabriel shocks instead of Bilsteins.

On the other hand, if there is any play in the buffers, you will get a thump when the lower one hits the frame.

Speaking of which, you DID use Bilstein or Boge, didn't you? If not, that is the thump right there!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!