An additional cam clunker is the brass thrust washer that goes behind it. When it gets worn it give the cam some fore/aft play. At higher RPMs the gear is in a constant pull (the oil pump drag for one) and the slack all stays one way pushed by the gear's angle. At idle, however, the valves can push the cam forward momentarily as they come past TDC on a lobe. This produces a forward/backward torque on the cam, and the gear angle can then makes the cam slide fore and aft. This also makes a nice deep knock that sounds like something seriously wrong with the engine. However, a rod knock doesn't disappear when the engine is revved, it can be heard at higher RPM's still when the when you close the throttle.

In my newest KgTrimning catalog I saw steel thrust washers. Probably not a bad idea to go along with a steel gear set. After all - it would be a shame to have to pull a 250,000 mile gear every 90,000 miles to replace a worn thrust washer.

High RPM's and high lift cams all put more torque on the valvetrain, and hasten the demise of the gear and the wear of that thrust washer.