2001 Oldsmobile Alero air conditioning trouble?

Okay heres the deal, Its kinda complicated to explain and for me totally confusing. Alright, my friends 2001 oldsmobile alero has an air conditioning problem. The AC doesn't work in the first place. but what happens is when the air conditioning is off, the engine makes a terrible noise like metal grinding with metal. When the air conditioning is turned on however, it stops. Along with if you turn to defrost or defrost plus feet the noise stops. Or if you turn it to the mode where it recirculates the air that is inside the car it also stops. This car is a pile of ****. But all the other problems (for the most part) I can fix for him but this one doesn't make sense to me. I have theories but I dont wanna go make him waste his money based on my theories. I could see it if it happened when you turn AC on if it made that awfull noise I could probably find the problem but what its doing makes no sense to me. BTW its the 2.4liter L4 Engine its not the V6.