A few weeks ago my distributor cover, the black plastic square shaped piece at the front of the engine, broke loose and was hitting the engine fan. I would check for anything hitting the fan, and also the auxilliary fan(s) up in front of the radiator.

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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out