Ring gear starter interference is entirely possible, or a weak return solenoid spring allowing the Bendix gear to interfere from not returning fully. One would think that if the Bendix was touching the ring gear it would be intermittent because the starter isn't going to stop at the precise point each time. I didn't pay close enough attention to the video to know if it's a GM direct drive starter, or a gear reduction mini starter.
I suppose that after the engine is running, loosening the starter to block bolts would gain enough clearance to eliminate the starter rubbing the ring gear. If I were looking for the noise I might just try that next, only take a minute or two to test that theory. I have seen starter shims end up touching the ring gear also when they are shoved into place, one shim will sometimes twist out of line from the others. Instead of straddling the innermost bolt it will twist towards the rear, contacting the flexplate inadvertently.
The only other area I would be concerned about is contact from the torque convertor inside the bellhousing area, but if you're going to repeat the disconnected torque convertor test, you'll cover that area.
From my previous experience I don't believe there is any problems in the lower end of the engine, and oil filter dissection has pretty much taken the bottom end out of the equation. Paul IMO, seems conscientious enough to make sure the engine was done correctly before he shipped it, plus he said no noise on the dyno so some other variable in the car that wasn't present on the dyno, stands likely to be the source of the noise.
