Squeaking, rattling sound and vibration when I turn on my cars AC?

When I turn on my cars (1998 Mercury Mystique) AC my car makes a chrip/squeaking/rattle-type sound and the steering wheel vibrates. When I turn my AC off, it completely stops. And it is only the AC, when I turn it to panel or floor or defrosters it doesn't make the sound.. Only with my AC. The AC works fine, it comes out cold. I had my belt come loose a few times before when I drove over rain puddles, could it be my belt? Any help would be great, I don't have a mechanic that I regularly go to so I don't want to be charged a million dollars for them to tighten a screw.. THANKS!