Knocking sound when ac clutch is on and in gear
 

1993 Saturn Sl2 Automatic

As long as I’ve had the car the ac never worked, the clutch never turned on. Since I’ve put ac in, the clutch turns on and the vehicle has ac now.

but since I’ve had it, I’ve noticed a hard knocking when the vehicle is in gear that stops when the vehicle is no longer in gear or the ac is turned off. At first I thought it was the transmission but it is coming from what looks like the idler pulley. When the clutch engages, it’s as if something is either loosened or tightened because you can see the bolt on the idler moving frantically.

is it a bad idler pulley?

Could be.