Since I've seen at least a dozen cracked flexplates over the years, it does sound like a cracked flexplate on that video. Pontiacs are less prone than chevys are, but I have seen both along with a few olds and buicks over the years.
It may not be a cracked flexplate, but the noise is consistent with one, and yes they are constant every crankshaft rotation. I've seen chevy impalas with 2 speed powerglides driven by senior citizens crack flexplates, the car doesn't have to be abused for them to crack. There is no rhyme nor reason why/when they crack, they do just crack for no apparent reason. It's actually metal fatigue that eventually finally overcomes the initial strength of the stamping.
I've seen them with small cracks and I've seen them where the whole center around the crank flange bolts is completely broken in 2 separate pieces. All that is needed a crack where the edges bind, and click against each other, and you get the noise. To someone that has little experience with engines it may sound as a rod knock, but it doesn't normally change with engine load as a rod knock does. It also is not as heavy as a rod knock.
One thing that confuses the issue is they have already removed the convertor bolts and ran it with the engine with the convertor pushed away from the flexplate. Since the flexplate has no stress on it when disconnected from the convertor, it should have gone away. So even though it sounds like a cracked flexplate it may not be that, given the detective work already done.
From the video, it does seem to be in the flexplate area of the engine, however sounds travel in a reciprocating engine and are not always what/where they seem to be. If I had to bet on it, it's not a flexplate, but the noise seems to be concentrated in that area of the engine/transmission assembly.
Pontiac did use a very fine/thin shim between the crank flange and flexplate on some applications around .010-.015 thick. If I remember correctly, I have left that shim out without any negative consequences, probably in this circumstance it would not cause the noise in question.
The noise does sound like it's from the sheetmetal accessories on the bottom rear area...………
