2013 Ford Edge AWD making grinding/humming sounds (HELP)

[RyanCooper]

So fairly recently my mom bought a 2013 Ford Edge SEL AWD with around 120k on it, seemed to have good service history and didn't notice really anything out of the blue, it had what sounded like some faint tire noise at higher speeds but didn't think much of it. well it seems to be a humming only under acceleration mostly noticeable above 40-50mph, turning makes no change, only acceleration, let off the gas and it goes away. noise appears to be coming from the rear, maybe rear diff? 

 

Then there's this terrible grinding sound, almost like a failed or failing wheel bearing, sounds like its coming from the front somewhere, turning sometimes affects it, mostly heard around 40-50mph. I'm hoping its not the PTU but maybe so.? looks like its been replaced at some point already (looks cleaner than everything else and has markings on it). Wheel bearings themselves seemed to check out okay, even had her take the car to Ford and another independent mechanic we trust, Ford didn't even test drive the car. and tried to sell things that weren't even an issue (like leaks that weren't there, and verified by myself and the indie mechanic). the local guy was simply stumped and called people he knew including ford techs and no one had any answers. id hate to just toss money at a PTU and rear diff if its not the issue, but seems like it is... as much as that sucks. I wish I could convert this thing to FWD to avoid dealing with the fragile AWD parts these had lol