What's with this turbo-type whine? (archive)

I hear a mystifying but kind of pleasing light "ZZZZZZZZZZZZZZZ" (rising in pitch with RPM) when I accelerate, seemingly from the back of the engine compartment or the trans housing area. It sounds midway between 2 familiar noises: a turbo unit spooling up, and a radio picking up revs from non-resistor plug wires. But no turbo in my car, and the noise occurs even without the radio on. I've only had the car a couple of months; heard it all along. Fluids are all OK and car runs great. Is this normal, and what is it?
Thanks in advance.

Follow Ups:

Had chips on the tips of my fan blade and bad>>. Lars PNW 82_528e 13:26 6/03/2001 (1)
- Now that I think about it, Lars is probably right. Tony - without a 5er for now 23:30 6/03/2001 (0)
Rear end maybe? Tony - without a 5er for now 23:34 6/02/2001 (4)
- Don't think so. It changes pitch with engine RPM Chris R '87 535iS 11:09 6/03/2001 (3)
  - Does it sound mechanical or electrial? Tony - without a 5er for now 11:20 6/03/2001 (2)
    - More elec., but I can't think what would pick up Chris R '87 535iS 11:37 6/03/2001 (1)
      - Fan clutch maybe? Distributor? Flywheel? Tony - without a 5er for now 13:00 6/03/2001 (0)

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