Trust me, you have a loose belt. Be aware that once a belt squeals, it becomes glazed quickly and can still make noise tightened. You could have 2 problems but you need to diagnose this in a logical order. Tighten the belt and load test the alternator. And be prepared to replace the belt.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.