Lately, when my car has been idling, I can hear a loud knock in my exhaust, seeming to be coming from the muffler. The other day I put it in neutral and let it idle and looked underneath at my muffler. Each time there is a knock the muffler moves a little. The exhaust line stays put, so I know that it is not an exhaust hanger. Also, every time this knock occurs, the engine moves to one side about a quater inch. Therefore, I know that it is something misfiring.This knock never happens when Im driving and my rpm's are up, just when I'm idling. I just had the engine rebuilt, so I know that it is nothing internally wrong with the engine. Anyone ever had this experience or have any ideas as to what it may be? If so, I'd appreciate your help.