3 Answers

I hope for your sake it is a valve, knocking sound coming from the motor is not a good sign, It can indicate a blown motor.You will have to ask this question to a mechanic.

Before you begin : Use the poor mans engine stethoscope . 36 inch dowel about half inch in diameter. Fire the engine up and touch the dowel to engine and the other end to your ear . Listen for the source of the knocks . Keep clear of fan and moving parts as this will not turn out well. For the moment the valves are just a guess this will really pinpoint the problem . I dont want to be the barer of bad news ,but valves may be wishful thinking . Extreme heat destroys engines fast . Considering time ,energy ,costs ,going with a new small block may be the most effective ,efficient way to go. You only wish to do this once so why not get it right the first time . Saves money in the long run . Best of luck with this .    Bill

Valves don't knock... you might have spun a rod bearing