Thanks, Channing. Drive shaft is a 2-pc unit, hanger bearing is new, u-joints are fine. The drone sound happens even when in "park". That's what is so strange to me.
Thanks for ideas!
It's an automatic. You think it may be torque converter vibration?
Unlikely the driveshaft if it does it at idle.
Hole in muffler. Not likely forward of it or would be loud tic sound.
Look for anything near the exhaust pipe or where the pipe would move and come into contact with the body.
Cab or hood loose?
Cab noise update:
Noise was coming from multiple areas. The problem while driving was happening at 45 mph+. We put the truck on the rack this week, and found a problem at the rear diff yoke. The u-joint cup was a loose fit on one side, causing the joint to work side-to-side in the yoke while driving. We tack-welded the yoke to hold the u-joint in place until proper repairs can be made. This solved most of the driving noise, although I still have the horrible drone at low rpm's whether driving or in park.
The tranny was replaced about 3k ago by the PO, and work was performed by a shop that doesn't have a really adept mechanic. The bellhousing has been welded on the bottom, and I suspect someone torqued the bolts while mis-aligned with the dowel pins and cracked the bellhousing. I am starting to suspect a warped/cracked flexplate or TC problem. However, she still shifts and runs 100%, so I'm gonna run it!