Update: Figured it out... kinda... It could be one of the two things I did. Either way, the noise is gone.
This truck originally had a BOSS plow mount on it. When I removed it, it made use of the tow hook locations. But they were longer to compensate - so when I took the plow mount off the bolt ran out of thread before it was tight and rattled. Somehow that resonates up into the cab.
That said - I also did look around the transmission for anything that could rattle and found only some of those heat shields that were a bit flappy, i bent them slightly to stiffen them up.
Needless to say - one or both of these led to the rattle going away. I doubt very much anyone else would have this situation but hey you never know.