Highish pitch whine at slow speed

[asymptonic]

Just today started noticing a highish pitch whine when at very slow speeds, between 5 and 15mph when the vehicle has been driven a few miles. Stops if the vehicle stops, or if you go any faster. Pitch is high enough that it doesn't seem like it could be wheel RPM dependent, and definitely not vehicle dependent. Happens in any transfer case gear, and happens if the transmission or the transfer case is in neutral. (NP231, AX15).

Got under the vehicle and there is a bit of oil on the bottom of the transfer case, looks like it may be seeping through the oil fill bolt. Planning on popping that out and topping it off but has anyone else encountered this?

 

[RCP Phx]

Check all your vacuum lines, particularly the one to the vacuum canister.

 

[asymptonic]

Should add it sounds more like a mechanical whine, and I wouldn't expect a vacuum noise to stop immediately when the vehicle stopped while at idle that whole time.

 

[RCP Phx]

Look for things that rotate regardless of rpm, wheel bearings, rotors, u-joints, etc..

 

[asymptonic]

Checked the t-case fluid, pretty much full and clear. U-joints makes sense to me. I greased everything under there and they took 4-5 pumps easy. Will see if that clears it up.