2002 Honda CR-V AC Causes Vibrations

What could cause my engine to start to vibrate when I turn on the fan for the air conditioner and stops when I turn it off? I feel the vibration in the steering wheel and, to a lesser extent, the front seat.

The belt that drives the compressor may be loose. Also when the compressor cycles on, the drag on the engine tends to slow it, and it takes a moment for the engine controls to speed the idle back up to its usual RPM.