Welcome to CHR !!!!! Header gaskets do have a nasty habit of sounding like a lifter out of adjustment....and a question, it only starts when the engine is warmed up??? What is your oil pressure doing from cold start to operating temp????? Could be something as simple as old, dirty, lightweight oil causing a bit of a pressure drop once the engine gets warmed....hydraulic lifters can get "loose" with low oil pressure..... Hard to say with no history on the engine, but a couple of things for you to look at, anyway....