AC runns off a pully that spins a clutch on the AC compressor, If it only does it when you turn on the AC it could be the compressor clutch?
I would look at it with the engine on and have a buddy engage the AC, look for the clutch to engage and the AC compressor to start spinning. If thats working normally you may have a 134a leak low or high pressure might cause weird noises. grab one of those AC recharge units with the 134a and see if your line is either excessively high or low.
Slow is Smooth > Smooth is Fast