The dealer is acknowledging the sound is not normal but cannot determine the cause so far so they are holding it over night. I am glad they are at least acknowledging it. Not sure what will happen next, but I'll keep you posted. As a reminder, this is the sound the car makes on acceleration: https://photos.app.goo.gl/H6nvXWdFA6dXfH1o7
Yes, it looks like reduction gear damage. Or a drive shaft joint or one of the bearings. The noise increases with speed, it's interesting to see what happens when cornering.
After listening to your recording, I would not call this a "low frequency hum". To me it is a scary hum. My car has developed a new noise, also related motor speed and annoying, but nowhere near the recorded noise. It is hard to hear unless you are on a recently paved road where the road/tire noise is minimal.