I suspect it’s the timing chain/timing gear, I replaced many over the years.
Pontiac used a nylon timing gear in those years and when they wear out and the chain stretches causing a noise that sounds like a gear drive. They eventually wear enough to cause the chain to jump and potentially wreck the valve train.
Open the timing cover and inspect the assembly. Replace with steel gears and double roller set as needed.
I know this engine has been rebuilt and I would assume that he would of upgraded the timing chain. Uncle built the motor so, I will ask him if he knows what's in it, if he doesn't I will definitely check that out. Thanks
