Engine knocking 2002 Cross Country

[peter507c]

with 84,000 miles on the car and the car running normally and well suddenly there appeared a loud tapping/knocking from the upper engine head area around #5 cylinder. Spark plug ok, coil changed to check performance. Very rough idling and very sub normal power. Tapping/knocking briefly disappears under load. Car has only been idled briefly after problem appeared. Timing appears normal as it starts readily. No smoking of engine etc. I am at the stage of removing valve cover to inspect. Any Ideas or experience with this problem