Engine noise/clicking
 

I've been trying to figure out an engine noise I have and am getting no where with it. The noise is random and sporadic and comes and goes. It sounds like a clicking noise in the valve area. It does not seem to matter if the engine is warm or cold because it happens at both times. When the clicking starts if I rev the engine it tends to click once more slightly louder and then stop but not always. I did loosen the distributor lock screw to see if possible the shaft was binding but seems to be little to no change, maybe slightly quieter but that's all. Any ideas?

Re: Engine noise/clicking
 

Worn cam plunger and/or spring allowing the cam to walk.