So I have a 2001 cavalier z24 everytime I turn on the the A/C it sounds like the motor is knocking, only if the engine is at operating temp. If not the rpm's are little higher and there is no knock. Now if the engine is at operating temp and i. drive and bring the rpm's to let's say aproximately 800rpm it stops. Pretty much anything that makes the rpm drop it sounds like it is knocking, the car runs very well and blows very cold air so i can't figure this out. To me it almost sounds like a diesel with piston slap anybody have a clue what this is?

I have the sorta the same thing... I was able to narrow mine down to a lose exhaust - the extra engine load from the AC causes mine to rattle pretty good sometimes