This started happening gradually and now happens just about everytime. If I'm at a stop and have the brake pedal pressed down while my heater is on (this morning did it while it was off though), my car will emit this extremely loud squealing noise until I let go of the brake or turn off the vents. Either of these two solutions fixes the problem, but it's really annoying that it does it at all. Any ideas? It was doing this a few weeks ago and then I had my accessory belts changed while it was in the shop, but it's still doing it.