Will Back

Yea, I had a look at both the TC and the flex plate, they both looked fine on the outside before I put the trans back on. Still haven't found anything yet, double checked that all the exhaust bolts are tight

Agreed, I had a good look under there and I haven't found anything yet, I need to poke around more still. So far I've confirmed the transmission cross member is good, has a new trans mount, exhaust y pipe is all tight, and center support bearing is tight

I recently picked up a 2006 Silverado 1500, the previous owner stated it was making a knocking noise. He pulled the transmission thinking it was the flex plate but didn't find anything. I just put the transmission back in and noticed it only happens when engine rpms are dropping. Additionally, it mostly happens when in neutral. Posted below is a link to a video where you can hear the sound.