Valve train noise can be affected with engine temperature and load. Too much lash will grow even bigger with engine temp up to normal, as compared to a cold reading. Also too much lash on the exhaust will sometimes tick louder under load as it has to open against increased cylinder pressure, but most of the time who can hear that when the engine is screaming unless you have a stock and quiet exhaust system, which i assume you do not. The valve lash can grow more with aluminum heads as the expansion rate of aluminum is so much greater than cast iron. Good luck on your engine and know that when you find out what was wrong you will be that much more educated and satified with your car and you will have information that you will be able to pass alone to the next guy! I would like to know what eventually fixed this problem!
that cast iron.