i recently rebuilt a 98 5.7 vortec, since day one it has had this bad knock (refer to video). played with valves to eliminate adjustment issues.it sounds like it is coming from the oil pan. Sometimes it will stop for a second when put in gear but comes back a little quieter. it gets either quieter or drowned out or stops when accelerating. ive checked my flex/converter bolt. all were tight with no scars and the plate looked good also. any ideas?

 

video

 

Sounds like a cracked flex plate, I had one similar, the crack was near the center so you couldn't see it until the trans was pulled.