What you ar hearing is detonation or ignition ping.
It is not the valves. Use a higher octane gasoline and or retard the ignition timing a degree or two.
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1966 Ninety Eight Convertible - 425 Tri-Carb, Factory Dual Exh, Switch-Pitch THM400, 3.54 Posi.
1966 Catalina Wagon - 421 Tri-Power, M21, 4.11 Saf-T-Trac, 8 Bolt Wheels
1989 IROC-Z - 5.7TPI
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