All we can do over a computer is guess, and my vote is for an engine pulley rubbing on a plastic splash shield. If you never hear the noise with the air conditioning turned off, look at the AC compressor clutch. If you hear it standing still like you mentioned, pop the hood and see if the sound will lead you to something rubbing. If so, suspect a collapsed engine mount that is letting one side of the engine sag a little.
The AC compressor will cycle on and off too but you really shouldn't be able to hear that inside the car unless something is happening to it. If the system is low on refrigerant, you might hear it flowing through the evaporator in the dash when the compressor cycles on. That is not real common but I have heard it on occasion. A better possibility is the radiator fan cycling on and off. You should hear that inside the car but the pitch won't change unless a bearing is going out in the motor.
You could also be hearing a belt idler pulley although the noise will typically be constant. The pitch will change with engine speed but it usually won't come and go.
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Thursday, March 29th, 2012 AT 7:41 AM