I have no idea. Only thing I can think of is when I put that transmission in, I kinda dropped it, and as well as mashing my left index finger, I think it may have caught on the flex plate, kind of bending it. I doubt it though, as it just started making the noise a few days ago. All the bolts were tight, I don't know what it was hitting or scraping against to make the noise, but said noise is gone and the flexplate was definitely bad, so... ¯\_(ツ)_/¯how the hell did you warp your flexplate?
Edit: Or perhaps it was bent when I got it (bought it from TA Performance), and it got worse over time?
I don't know. The engine shaking, moreso than usual, is gone now, being as Buicks are externally balanced.