i have a strange knocking noise coming for my xj engine compartment, heres the catch, it only makes this noise when the ac is on, occasionally without ac on. its just a random knocking, no rhythm to it at all.but, i can have the ac on and it wont make the noise while im driving, only idleing..need help! i hate people starring for all the wrong reasons!

 

Check the torque converter bolts and check the flexplate for cracks.

 

I'd check the idler pully as well (probably first as it is easier to get to), just grab it with your hand (engine off of course) and try and wiggle it, it shouldn't wiggle much if at all.

 

sounds like mine did found flexplate was cracked

 

I hope you fine loose bolts or a crack flex plate or your air compressor noisy - if not with the brake on and belt off - have some listen for engine knock - try a heaver engine oil might help for a wile. Have had air conditioner compressor knock when they are going bad.

 

Also check make sure your exhaust isnt loose and not hitting anything, if everything checks out then check the torque converter