I am having this same problem just noticed it today while backing out, and I only have 700 miles on my GC. I wonder what part was greased as it sounds and you can actually feel the hummmm while in reverse.
Both my 2014 and 2015 have done this. It's kind of a humming/rubbing noise in reverse that you can sometimes feel too. My dealer service manager and I went and tried out a bunch of 2014/2015 WK2s, new and used (he was curious too) and about 50% did it to varying degrees. My 2014 did it worse than my 2015 does. It seems to be only the 4WD versions (unless someone here can say otherwise) and our guess was that it's the clutch pack in the transfer case and is probably normal for operating in reverse. That's as far as our investigation went though as it didn't seem harmful...just weird.