I was having the same kinda' sound except mine seemed to be more noticeable when I was sitting at a light somewhere. One thing I found was causing a rattle was that the heat shield for one of my catalytic convertors had come loose and it was sitting against the convertor and would rattle from time to time.

Might be something to look at.

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Frank
84 500SEL EURO 101K ( JUST LIKE MY 1ST WIFE. GLAD TO GET RID OF HER! )
85 300D 310K (sold)
90 350SDL 184K sold
83 300D 118K (sold)
88 300E 153k (sold)
93 400E 105K (sold)