Best thing you can do is get rid of the u-bend. I recommend either cut it out and straight pipe that little section, or headers especially if going turbo build. On my stock GTP I got a ZZP 2.5" catted downpipe and saw knock only on wot pulls when it shifted. But then I went the cam route and got headers. Just get headers right off the bat or delete the u-bend after the cat and you should see a lot less knock