I don't have the W124 wagon, but my 560SEL used to have that noise when go over bumps or cracks in pavement.. it goes like "kaboomm". I found out the upper strut bushing were worn.. replaced and the noise went away.

So check your upper strut bushings..
Goodluck

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1991 420SEL Project Car (Completed and rides beautifully) http://www.peachparts.com/shopforum/showthread.php?t=112321
2000 BMW 740IL
2000 Integra Type R - Track Car - SOLD
2005 S2000 - Track Car
06 C230 Sport
89 560SL