Valve Tap

I have a 1998 Dodge Stratus with a 6 cylinder (2.5L) engine. It has just over 90,000 mikes on it. Just recently it developed a valve tap, but the tapping comes and goes. It usually taps when the engine is idling, however, if I raise the rpm's the tapping goes away. I tried flushing the engine and changing the oil. That did'nt help. Any suggestions would be appreciated.

probably some carbon build up around the seat.