[UPDATE 5/2/18] I've confirmed the rear drive axle engagement that causes the "hum" is able to be prevented by turning off traction control. I'd be fine with the hum if I hadn't already heard from 2 sources that the rear wheels / 4wd should not be engaging at speeds higher than 40, yet it happens often on my compass, so when I hit highway, I turn it off. I have more questions, but I think they're best for a Jeep tech. If I hear anything else I'll post up
I heard it wind up and hum today on the way to work, this time going well over 50 mph. I'm about ready to type up an email and send it out to a few places, including a dealership, Jeep corperate, a few online Jeep enthusiast sites, etc. I'm still fairly certain the humming we hear is some kind of system doing it's job / something normal, but it'd be peace of mind to confirm this before more time passes.
I dug deeper. I read the renegade forum threat, (50+ pages now!), and I thought there was one post in particular that might put some of your concerns at ease, at least for the time being...until warranty expires lol. Basically, half way down the page, there's a fairly large post describing what is happening, and why. In a nutshell, it's a bit of what I had mentioned earlier, with the 4wd system prepping itself for potential use, which in turn, is why we hear a spool up and humming sound at various speeds and temps.
This post even touches on why the various suggested methods to eliminate it work, like turning off TC, etc.