Well, wish me luck, my C-Max is in the dealership this morning for this same issue (along with replacement of the door latches under recall). The noise started a few months ago for me, and has gotten worse. The best way I can think to describe it is that it sounds like a stone wheel on a stone axle. Noise starts at 10mph. It gets louder at higher speeds, but never changes pitch. I thought it was the tires at first, but a new set didn't fix the issue. When I dropped it off, dealership said they were going to charge $100 to diagnose the problem. I'm not sure they realized that the trans is still under warranty (2013, 66k miles). I didn't correct them... maybe I'm just paranoid, but I feel like I may get a more honest diagnosis if they aren't aware that Ford could be footing the bill.