1988 toy 22re, the motor has a knocking noise coming from the head (#4 cylinder). we pulled the valve cover expecting to see a broken rocker arm but everything seems to be ok. the engine runs smooth with no miss. it did run hot last night for some unknown reason. that is when the noise started. the noise sounds like it is coming from the head not the bottom of the engine. could it be a warped valve? anybody got any suggestions for a check list to find out what the problem is?