[chiphead]

Sounds like it's once per revolution. Could be something hitting the windage tray. Could also be an endplay problem. Could be the fuel pump or eccentric coming apart. Could be an issue with a belt driven accy.
Since you have a good relationship with the builder, call him first.

Here is what I'd do:

Cut the oil filter open. If you find metal in the oil filter, it's game over. No reason to do anything else, pull the motor time.

Remove fan belts to eliminate an issue with the water pump or power steering, alternator other belt driven parts Run engine just along enough to confirm noise is still there. Remember that water pump will not be turning.

Next try to pry balancer in and out and confirm the end play. Too much end play indicates a main bearing problem. You may need to put a dial indicator on it because proper end play is a very small amount. Verify balancer is not coming loose.

Next, pull plugs and try to turn the engine a couple full revolutions using a breaker bar. Listen for the mechanical tink or other noises you roll engine over by hand.

You may have to pull fuel pump, or timing cover to make sure that the timing chain and fuel pump eccentric are not coming apart