you didn't mention how the rapping sounded. does it change with the rpm. does it do it only while moving or does it sound off at a stand still? does it sound off on acceleration or deceleration. have you ruled out the engine compartment?
i am replacing a camshaft this week on my 420sel. i would describe that as a sharp rapping also. mine lunched a rocker arm wore down so bad i have to replace the camshaft as well.
with a bit more description we (the forum) may just figure this one out.
1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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