2017 F550 Very Loud "Rotational" Buzz/Rattle Under Truck

Hi all,

I have a new-to-me 2017 F550 6.7 Diesel 4x4, with 91k, fitted with a Versalift boom. I am looking for help identifying what I think is an abnormal noise.

After the truck warms up, there is a very loud buzz/rattle noise that seems to emanate from directly beneath the cab. The noise is loudest at idle and seems to become quieter with RPM, but I don't think it goes away entirely. I took this video; you can hear the sound quiet some when the truck's idle control kicks in, or maybe its just the engine noise drowning out the buzzing. The noise sounds more "rotational" than loose part rattle to me.

What I've tried so far:
> I have crawled around poking and shaking things, but I have not found an obviously loose part.
> The transmission dipstick is mentioned as a common rattle on these trucks, I have checked this, I don't believe it is the issue.
> I have shifted the transfer case in and out of all functions, no change in sound.
> I have shifted the transmission in and out of all gears, no change in sound.

Any ideas what could cause this noise and if it is something I need to be concerned about?

 

Hot ####! Thats some destruction. I wondered about t-case. I figured I'd start by dropping the fluid and see if it's sparkly.