Little update on this, did some maintenance on my truck. Replaced some fluids (diff/tcase), removed the brand new fram xg2 and installed a mopar mo339 (don't shoot me please, I know its not ideal, its there for another test), inspected the engine air filter and tightened the 2 worm clamps on the air box/conduit leading into the intake.
I could barely turn the fram off by hand, broke out the pipe wrench so the mopar replacement I put on a little looser and only used one hand to tighten it back up. Feels like I should go a little further with tightening but I refrained and will look for drips.
Anyway, sound is almost gone? It seems to be reduced severely both in volume and the amount of times I hear it. Have to really listen hard now.
So I guess something in there that I fiddled with is connected to that sound. The air filter box I was really careful this time making sure it's all seated and the screws were tightened/torqued in staggered order (probably not needed but hey), the two clamps on the box are a little wonky, I swear I could turn them further but they're just worm clamps and I'm worried about busting something, feels a little cheap and weak to me but what do I know.
My guess is, the air box? Either that or maybe the oil filter was too tight and causing some resonant frequencies on the aluminum adapter (if the gasket was pressed tight and the filter was directly up against the adapter with no gap left) but that's got to be a pretty big stretch I think.
I'm out of ideas. Just happy I'm making progress!