Wild guess!! seat belt reminder buzzer? the inner buckle should have wires attached to it so once the belt is firmly inserted, it silences the warning buzzer. So look at the inner buckle next to the console and under the seat for pinched or broken wires. And all this is assuming you wear the seat 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.