I have a feeling that "knock" is something common to the newer HEMIs. A while back I started a post for a friend that has the same issue. He still hasn't taken his in. His is a standard cab, short bed, 4X4 with an 8 speed auto. He doesn't do any towing with it, but it doesn't seem to lack any power. It'll darn sure get on outa here when you step on it.