Low vibration with air conditioner on

My 2002 Nissan Altima, 4 cylinder, 140,000 miles, has a low vibration “RRRRRRRRRRRRRRRRRR” from the engine compartment with the air conditioner on, the car in gear (brakes on)at 750 rpms. With the car in neutral or park, the rpms go up to 850, and the vibration stops. There is no vibration when I am driving. Any suggestions?

I had a similar thing with my Saturn, and it was a loosened belt in the AC itself. You should be able to see if something has swung out of alignment under the hood.