2006, 5.4L. Theres a weird knocking sound that you can hear at idle and sometimes while driving, almost like something tapping on the firewall. Its not consistent though, some days its there a little bit, some not at all, and some it won't go away. It seems on warmer days it is worse. Under the hood you can feel a slight bump but its not as prominent as in the cab.

At first I assumed something loose like exhaust but it doesn't seem to be the case.