It sounds like a problem I recently identified and fixed. I was getting some clatter from one rocker arm. It used to get louder as the car got warmer...but it really wasn't affecting my power. But I could definitely hear it. So I pulled the upper manifold and valve covers off and adjusted my valves (stud mount). I found two valves slightly out of a adjustment...but the thing that was making the noise was I had a pushrod that was clogged! You know where the oil travels up the middle? Some gasket material or some RTV silicone crap must have gotten clogged in there and wasn't allowing the oil to come up to the roller rocker. So I replaced two pushrods, used some lapping compound to smooth out where the pushrod meets the rocker arm...and all is fine now.
I hope that's not it...but I thought I'd share my story.