I have this weird noise that comes from the engine compartment and you only hear it when the A/C compressor is running and the gears are engaged while braking. It sounds like a low whirr that goes on for a few seconds. I noticed that if I shift into neutral and engage the brakes while the A/C is running the sound does not come on, also if I have noticed that there is no noise if the A/C is OFF. Could that be the compressor? the idler pulley? the belt tensioner??
Appreciate your time