This is whats called a "throttle sensitive" noise. So I would look at air cleaner sealing, possibly a wheel bearing, or a belt. Hard to determine over the internet!!! Make note of what RPM as well as road speed that the noise starts and try to duplicate the noise sitting still, bringing the engine up slowly to that RPM, then feather the throttle. Or when it starts, put the car in neutral while driving at that speed to see if its engine speed sensitive or in fact throttle sensitive and report back.

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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.