If the flexplate is a B&M, then it cannot crack like a factory one, and should be good.
I really don't think that sound is internal.But I could be wrong. It doesn't seem to be muffled.
Maybe, just for the heck of it, check the crank end-play.
Also, might want to check with the stethescope, the engine oilpan, above the liquid line; maybe the windage tray is singing, or rattling on the pan.
