I have a 2006 duramax that has a knock at 1300-1700 rpm only when warm. No white smoke and only mods are a k&n filter, and turbo back exhaust. Also have a bullydog programmer on Eco tune. It runs and drives fine. Started about a week ago when I was going up an incline. Been doing it ever since. I have never towed anything with it since I've owned it. Any advice or ideas would be appreciated. thanks!