[Spiegelglatt]

Not to get your hopes up, but I had a noise exactly like that and took months to figure out. But my noise was due to the Vertex solution having a thin amount of grease between the main bearing and aluminum casting. The bearing would then rotate inside the casting and make this exact noise (cleaned with brake cleaner and is now gone). If you don't have the Vertex, perhaps center main bearing itself...not the support I am talking about the bearings themselves. Also, it might not make noise on lift, because driveline tension would be gone, but a stethoscope to the main bearing area should still render the noise at a decreased volume...this is similar to how I found mine, but I put my phone on record and then placing under passenger's seat and there the noise was most pronounced... hope this helps...mine did sound like wind or like something stuck in the dash vents, I suspected this for the longest time...

 

[Spiegelglatt]

Wonder if a wheel bearing could be the culprit?

 

[Spiegelglatt]

Couple odd questions: can you pinpoint where the noise is coming from driving left / right / front / rear or no direction? If you can't tell direction than it is most likely center...
Does turning left or right increase or decrease it? Does braking do anything to intensity or pitch?
Wheel bearings in my experience will change with turning left or right by either diminishing or getting louder...they also seem to need the weight of the vehicle to make noise so it would make sense that it wouldn't make it on the lift...

 

[Spiegelglatt]

Excellent! I think you would know right away if the alignment was off because it would shake / have a vibration. Sounds like things are all good.

 

[Spiegelglatt]

Perhaps time for a lift such that you can listen underneath while noise is replicated. I have read of differential noise that some have had, does this sound likely? I do not think the OEM shaft bushing would be capable of making a rumbling noise...perhaps because it isn't OEM? Shaft wear sounded like a squeaky bird for me because the bushing had disconnected from the bearing allowing the bearing to somewhat freely rotate. Later this was replaced by a shuddering shimmy, then an all out flat tire feeling. I can not tell you the exact measurements on the amount of play (perhaps someone else can), but both my trucks are pretty snug when trying to wiggle the prop shaft by hand.

 

[Spiegelglatt]

Re-packing will certainly help. Yes it sounds like a slipping belt and is almost non-directional...hard to tell were it is coming from because it is right under you. I am have not done this with the factory bushing still in place...I am not sure if clearance will be an issue here (I had my bracket and rubber stripped and cut off the drive shaft at the time because I was installing the Vertex solution). Procedure is otherwise simple...there are two seals and my bearings were pretty dry, but looked good otherwise. It is funny because this support really doesn't take much weight....yet the bearings are pretty big...