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2008 impala whir in engine compartment when A/C compressor is running &brakes & tran. are engaged

Hello again

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

I’m stumped.
Try picking up a mechanic’s stethescope (less than $10) and, with the parking brake engaged, the tranny in neutral (manual) or park (automatic), and the engine idling, carefully touch suspect areas and see if you can locate an unusual sound. Be careful in there. Pay attention to th eprobe and don;t get it cought in a belt.

Lots of unknowns here and with these type problems it does help to know what type of vehicle we are talking about. A pure guess, you are probably hearing the clutch possibly slipping as the AC is engaging/cycling on. At higher RPMs, like neutral, the slip isn’t heard as much and in some cases might bite quicker than the lower RPMs as backwards as that sounds.
Give some more info on the vehicle type and someone here can probably offer up a better suggestion. Miles on the vehicle and type of driving, city or highway, can also help give you a more accurate description of what is happening.

How about telling us how old your unknown make and model is. Auto or manual transmission, and how many miles

So sorry you guys, i thought I filed it under the same inquiry of my 2008 impala SS. its fot 51530 miles on it as of 12-24-2012 and its got automatic transmission. I drive mostly highway with up to 20% city driving, I am pretty aggressive though, especially when cut off…lol