Is this rodnock?
 

that’s a video of the sound my brothers 96 impala is making

we’ve been told it’s the flex plate but we unbolted the torque converter and slid it back enough to get a good look and dont see any cracks at all . It seems like it would have to be a pretty big crack to make a sound so loud

He just bought this engine and trans from a guy known as “impala mike” whose supposed to be a reputable impala/caprice parts supplier . He says it ran fine before he shipped it to us. Supposedly has like 70k miles

what do you guys think it is and where should we start?

Hard to tell. The speed of the knock seems as if it would be, but doesn't sound deep enough. Sounds more like a playing card on the spokes of a bicycle. Have you checked the oil for bearing material? What you can do is take the heads off, rotate the engine by hand, let a piston go on the downstroke for a 1/2" or so and try to push down on the piston. If the piston pushes down you have discovered the source. There should be no play with the rod/piston during reciprocation. You didn't find any sort of markings on the flexplate as well?

I dunno. If it is bottom end, you've got more than one knocker. The frequency is too quick for just one rod.