I'm thinking the noise was always there and you couldn't hear to until the muffler was added. I'd check out what Tom suggested about the pushrod clearance. Did you add pushrod guides with the roller rockers?
Tom are you saying to pull the cam to look at the bearings?
No It wasn't an exhaust leak. It was something the exhaust was doing with to tube header and the cast header quieted it. It ran better with the tube setup, Old Viper header.
