jrothe, would you maybe describe your noise as a rattling noise (don't know exactly what knock means since that could be a deep and loud sound or a light and tinny sound - mine is more the latter)? I have this issue now, its definitely up front, and I can't track it down. Same thing, though, it only happens over bumps and I can only hear it if I have the window open because its not loud enough or heavy enough to be heard or felt inside the car. I thought a loose exhaust component, but everything down there seems to be ok.