Thanks to all who contributed to this thread. I started getting the whine noise a few weeks ago. I stumbled into the PTU issue, and replaced the fluid in that. Still, the sound remained, but would stop when I would get to around 40 mph. I NEVER thought of looking at the Intelligent AWD display to troubleshoot. As Pikuoff reported, I could track the sound to whenever power was sent to the rear wheels. The fluid change of the rear diff didn't help. Next step is to inspect the driveshaft center support bearing.
Until I get my GoPro set up to film under the car, I had a follow-up question. Does the driveshaft always turn, or it is controlled by the PTU? And if so, is it safe to assume that the rear diff isn't engaged until it receives a signal to engage? (If it always spins, then if the center support bearing was bad, it would reason that the noise would always be present.)
Thanks!
