Great thread. My M923a1 does the same thing, I just changed the oil in the transfer case and it was black. But didn't fix the noise. I have also greased all the drive shafts, still no change. Mine is intermittent and seems to be related to bumps in the road. The noise starts instantly and stops instantly, kind of like a compressor kicking on and off,it has no pattern. Also maybe thought it was the fan clutch, but I could never get it to come on with the hood up. My truck was a 2010 Red River rebuild with only a few hundred miles since rebuild.