My 82 CJ7 has developed an engine knock, or more of a clack. While on vacation, I allowed my roommate to drive my Jeep, as long as he promised to add oil every time he drove it, as I have a leak from the drain plug. He ran it pretty low and it developed a horrible sound, which gets louder as the rpm's increase. When you feather the gas it gets really loud as you let off of it. It's a 151 4-cyl with about 20k on a rebuild. I checked the rocker arms and the rods, and all the valves seem to be in order. Any ideas? Also, it doesn't have as much power as it once had, but again, only when the rpm's increase.