First time I turn the fan on (without the A/C compressor running...checked and double checked this), the engine RPM dips briefly, and the car makes some sort of a brief grinding noise. It's only done this a couple of times. Sounds as if a clutch is on its way out, but I didn't think the fan for the vent was on a clutch. Isn't it just an electric motor? Any ideas? I've only tried it turning the fan from 0 to 1. I was thinking it has to do with the climate control door(s) moving into position when you turn the fan on. 32k on the odometer, and its definitely coming from behind the passenger side dashboard.

 

If you have it in the defrost mode the A/C compressor will come on, and the idle may change and adjust.

 

No, not in defrost or recirculating. So far I've had it on the floor. This isn't just the compressor turning on, although today I'll make sure it goes on to compare the noise.

 

Ok, I just read the original post again. I bet you may have had a leaf or something in the fan, sometimes mice like to hide stuff in the HVAC case. Pretty gross. The RPM change could be from the extra draw on the alternator when you turn the fan on.