I had what I believe the be the EXACT same sound as you describe it. Mine also came and went and was never identifiable enough to get it to the point where the dealer would be able to reproduce it.
I say "had" the problem because yesterday the transmission gave out. It erupted into an extremely loud grinding noise and warned my wife to pull over and have it towed (which she did). It was dropped off at the dealer last night with the failure coming at 62,432 miles. Unfortunately I am now getting the official run-around from them. They agree the CVT is broken to some extent, but they are claiming that depending on WHAT broke and HOW it broke that it MAY not be covered by warranty. Of course, there is a lot of data on this forum (and in the warranty manual) to support otherwise, but it looks like I will have to dig in for the impending battle. I digress.
I did FINALLY mange to get a recording of the sound in question and have posted it in the link below. NOTE: YOU MUST LISTEN TO THE CLIP WITH HEADPHONES OR EARBUDS!!! You will NOT be able to really hear just how bad it is with a standard PC speaker. The moan/drone is a low frequency and is not well represented on something like a PC notebook speaker. When you put the headphones on, the moan is deafening - trust me.
Anyway - the only time I could reliably reproduce it was when it was in near freezing conditions which were VERY hard to replicate here in Texas (I imagine the same for you in AZ) so I know just how difficult it was to pinpoint this issue.
As a side note, my father is a retired Ford Powertrain Engineer. I spent a good part of my youth "assisting" him with identifying Noise, Vibration and Harshness (NVH). He would shout out commands to me and my brother in the back seat, "Where is that coming from?! Which side?! Lets see if how it changes with speed... Can you still hear it?!" It would make my mom nuts and I am now permanently cursed with "hearing" all sorts of things in cars that most consumers wouldn't even notice which, in turn, now drives MY wife insane. Thanks dad!
Link to Video/Audio File:
EDIT: Link to Video/Audio of transmission sound just after it happened (again - use headphones or a good speaker - phone or notebook speaker won't do it justice):
Edited by Ed Venture, 09 March 2018 - 08:38 PM.