For years now, when the AC is on and the car is idling, I get an occasional loud knocking sound from under the hood. Sometimes the car even jerks with the sound of the knocking. As soon as the car starts moving, the noise goes away. Of course…the noise never happens when I take the car in for repair.
Jerking + noise makes me think it might be a engine mount.
Thank you, but how would that related to the use of the air conditioning. This only happens when the AC is on and the car is idling, like at a stop light.
Try sticking your head under the hood when it’s knocking and see if you can determine the source.
Since it only happens when the AC is on, perhaps the sound is coming from a failing AC compressor bearing. That could be adding an erratic load to the crank that could be causing engine shake. My theory is out on a limb, but your ear should confirm or invalidate it.
Will do, thank you!
The AC does put extra load on the engine - so it is working harder and moving & shaking more. If you did have a bad mount that could - theoretically - be doing some thumping. I assume that is, at least partly, MikeinNH’s idea. Mtnbike’s idea is also plausible.
Unfortunately it will just take someone who can look things over when it occurs.