I drive a 1995 E300 Diesel and am encountering what I can best describe as an electric humming noise which happens when the airconditioning is operating. The noise increases in pitch with increasing vehicle speed. When I switch the airconditioning to "economy" turning off the compressor, the noise goes away. My alternator belt appears to be tight and there are no other apparent electrical issues with the car. I would appreciate any hints or clues about what the cause of this may be.

Best regards.