Hi, 98M3.I'm not a certified BMW mechanic (I'm a physician who owns a 99 M3), but I CAN give you a few reasons that the chirping in your (and my) car is related to the brakes: 1. The chirping is not related to a perception issue or anything in cyberspace, the fan shaft is either under lubricated or has an old worn plastic bearing and squeaks at just the right (wrong!) Speed. When the brakes are applied, so are the brake LIGHTS and they will steal a small amount--but enough--current from the fan motor to slow it down enough to resonance frequency (the frequency human ears can hear the defect."Chirp"). 2. The brake system vacuum line could be involved. Any number of complex feedback systems in these over-engineered cars may not be opening an air dam all the way or stealing vacuum from an air inlet and changing the afterload on the fan--thereby causing a chirp. Any bad vacuum lines on your M3? This last one is really doubtful--I understand that the baffles that open and close various air vents onthe car are electric-servo controlled, but you never know--THEY might be malfunctioning. I had a German car years ago that had a brakes-caused-weird-problems. And it was the bad vac lines.
Well, i've got the same chirp as do many others. From the answer you got from the above BMW "pro" I'd say we both need to keep looking for the fix. (I'm done with BMW dealer service, BTW.)
Tuesday, August 3rd, 2010 AT 5:42 PM