There has been much written about this. If you were to search a/c stopped, or climate control you probably would get a ton on it, but I will answer the sanme question again.

It sounds like the belt is slipping or the compressor is trying to lock up. With the engine stopped turn the front of the compressor by hand and make sure it turns smoothly with no metallic feel. If so, then examine the belt and tensioner.

Good luck,