so ive got a 496 in my nova, was out running around the other night and i hear a knocking noise, oil pressure was good wasnt over heating or anything but i shut it down and pulled the valve cover off and there was a rocker arm just setting on top of the head, i put it back on and it still seems to only be running on 7 cylinders, i cranked it over to make sure all the valves were going through their cycle and they are. anybody have any idea what might have happened?