That sounds mechanical, like a rocker tap from excess valvetrain lash. Could be a bent pushrod, failed lifter, or loose rocker.

I'd pull the rocker cover from that cylinder bank and wiggle-check each rocker. The bad one will be visibly looser than the rest. If you don't find one, whip out your torque wrench and 8mm socket, set to 22 lb/ft, and verify each rocker bolt is tight. Any bolts that turn before the torque wrench clicks should be noted.

The LS1 rockers are notorious for puking their needle bearings. I'm having mine upgraded to Comp roller trunnions over the Winter.

Turned out to be a blown header gasket.

I'm very glad to be wrong on this one.