My 1998 E300 (102,250 miles) has developed an interesting knocking sound from the rear. The sound usually happens when I am accelerating from a stop and is most audible with my window down. In fact, if I am next to another car, the tapping (like a noisy tock tock tock which increases with speed) echoes very loudly. This is definitely an "under load" sound but I don't know what part in the rear can possibly make such a sporadic knock under acceleration. Could it be a flex disk or driveshaft issue? It's more of an annoyance than anything else, but I would still love some opinions.

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1982 300D-T (sold)
1988 300E (sold)
1994 E320 Wagon (sold)
1995 E320
1998 E300D-T
1999 E3204M Wagon
2001 E320
2005 E320 CDI