I'm pretty sure it is the compressor...at first I thought it was a fan, because it sounds like a little brrrring noise almost like if a fan were on and it touched something. This only happens when I have the AC on, when the heater or anything else is on, it is fine. And this only happens when the car is rocked forward or backward...as in if I hit the brakes, or am idling around... On the interstate it is fine, and rather acts normally no noise, cools great, otherwise it cools great on the street I just turn it of cause of the damned noise.... What is it?