[panhandler62]

A loose rod doesn't (generaly .. ) cause vibration. It also makes a sound that is closer to a tick (not that different than the sound of detonation except for being very, very regular) than a "knock."

A tweaked pushrod will make one heck of a clatter as will a broken valve (and this one WILL cause a good deal of vibration.) You can figure out if a valve let go real easily by just removing the valve cover and giving things a good going over. A tweaked pushrod might be found this way also (though they can hide from this sort of inspection.)