Grinding noise, increases with speed under load

[NGAneer]

Let me preface the description by saying I've searched and even thrown some parts at this problem already.

I have a medium/high-pitched grinding or whining noise from under my 07 4x4 Yukon. The noise varies with speed (faster = louder) but not RPM. The noise almost completely goes away when I'm off the gas. At 70mph the noise is very noticeable and does not get louder with throttle input. It's the same with 25% as it is with WOT at a given speed. The only difference is if I get off the gas, then the noise seems to disappear.

From a stop, I can go WOT with no noise, it's only as speeds increase. There is no noise in Neutral while stopped or at speed. There is no noise with the trans in Drive and the transfer case in Neutral while stopped.

At this point I'm thinking that the noise is somewhere downstream from the front output of the transfer case but I want to try and get some help here because I'm getting super frustrated at this. Appreciate any help y'all can offer.

Things that have been done...

1) Rebuilt trans (for other reasons)
2) replaced rear axle bearings
3) Changed transfer case fluid (Autotrak 2)
4) Changed rear diff fluid (Lucas 75w90)
5) inspected inside rear diff (no obvious play in pinion bearing or extra backlash)
6) changed U-joints in rear driveshaft

 

[Foggy]

It sounds to me like a bad ring & pinion.. that's typical behavior when the
ring gear goes bad. Could be other things, but that just my opinion without going
for a drive with you

 

[tom3]

Had a Jeep GC that had a similar deal, the OD unit in the transmission was the problem. Didn't tear it down but replaced the whole trans.

 

[swathdiver]

I'd also start with the rear axle. Have a Go-Pro or a mount for your camera so you can stick it down there for a quick recording?

 

[donjetman]

Like Foggy said "sounds like ring and pinion".