Take it to the dealer. I had the EXACT same problem and they said it was the "Final stage resistor" or something like that. I have the work order but not on me right now. Lemme know if you need it. BTW this is definately not something you should try fixing yourself. My dealer had fixed this numerous times before on other e46's, talk to you dealer, they should do it for free. After i got my car back it hasn't happened since, works good as new now.dorem said:when the fan is on at low speeds, it starts whistling. I think I either have a bad motor or there's something stuck in there.
Any suggestions on what to take apart to look?