You didn’t say how many miles on this engine, it would have been helpful to know. You could have worn out lifters, rocker arms, pushrods, or camshaft. Or it could be a combination of wear no two or more of them on any intake or exhaust. I assume this engine has hydraulic lifters so it may not be easy to diagnose until it will make noise in park but all these things are replaceable by anyone who can swap engines.