I don't use the a/c very much, but I have noticed that when the a/c is on and I hit the recirculate button I will hear a noise almost like a fan starting up(duh) but very noisy, its a bit of a stuttered whine, its a strange sound. Then if I turn off the recirculate, I get the same noise again. The only thing I can think is that it is the sound of the mechanism that closes off the cabin from the external air moving back and forth(or whatever it does). I am pretty sure that is what it must be, but I am curious what kind of mechanism is used that could make this kind of noise. It is definitely not a pulley wheel/clutch on the compressor or anything like that, this is definitely coming from the dash area, and is affected by the toggling of the recirculate button.