i'm having an issue w/ my AC. the compressor may need to be replaced. i have sort of a cricket noise/whistling when the ac is on. there is no relation to braking though. my mechanic says that's between $250-700 to get fixed. it's been a while since i talked to him about it, but there is a chance it could be really simple and actually possibly be as low as $120 to fix. but if they have to replace several parts and do a flush of sorts it would be more expensive as it is a time consuming ordeal.
try this: start your car, stay parked and roll down your windows. turn on your ac. if after about five seconds you do not hear the cricket noise, toggle your ac off and then on again. wait another five seconds and if you don't hear it...it might not be your ac.
when you hear this noise are you driving and your windows are up or down? my ac can only be heard with the windows down.
if it's only when you're braking, it's brakes. It most likely means that there's just a little bit of grit in your pads somewhere and it makes a little chirpy noise. Nothing to get concerned about. if it's your A/C, then the chirpy noise is the belt slipping, so you have a) a loose belt which could be caused by a failing tensioner, I don't remember if the A/C is auto tensioning or not, I'll have to consult mr. Bentley on that. b ) Your a/c compressor is siezing and the chirpy noise is the belt slipping because the compressor won't turn the way it used to.