When going over large bumps in the road the car has a very definite THUD coming from the rear suspension. At first I thought it was the spare tire not secured, but that isn't the problem. Other than this sound, the suspension appears to be in good condition. Any hints??

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold