My 81 300CD just yesterday developed a whistle noise from under hood at about 35 mph. If I put it in neutral, the noise quits? Its not a belt squeal, more like an air whistle. Climate controls are set to ECON so no A/C compressor engagement. I could recreate it 3 out of 5 times at 35 mph and 3 times it would quit putting it in neutral. So I know its engine RPM related. Louder driving next to a sound wall as I leave work. Any ideas? Its rather difficult to have someone riding on the fender at 35 mph to locate the noise And I cannt get it to do it revving the engine.

__________________
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.