Hey, Sloregal, how much knock was there?? Usually if it is an exhaust leak you can hear it more at idle especially with the ATR headers since there is no metal sparkplug protectors to deaden the sound at idle (isn't that what you run??) After you check for all those exhaust leaks take the valve cover off and make sure its not a wiped cam lobe or anything. Good luck, I bet its something simple.