Intermitent valvetrain noise?

I have a ticking sound that I hear when the the engine gets warm, but it dosen't do it all the time. Could this be a lifter? Last time I had the engine apart I found a stuck lifter and replaced just that one. But now the engine is apart agian so I was debating repalcing all the lifters. What else should I inspect in the valvetrain?

Had the headers off twice with new gaskets so i know its not from there. I can hear the "tick" coming from the valve cover. I have the engine apart right now and I can't compress the lifters.

How long do they hold pressure and are they suppose to be able to compress after oil pressure is gone?

How much valvetrain variation can hydralic lifters compensate for, before shims or diffrent size push rods are needed?

thnaks