I have a similar issue: 2015 XLT, 21K miles, 2.5 auto, bought used from a Ford Dealer 3 months ago. "Thunks" when accelerating to pass or when coasting, then accelerating. Still under warranty & haven't had time to take to Dealer yet. I'm a former professional mechanic, it feels to me like a bad motor or trans mount. Also, transmisiison seems to take longer than normal to engage when shifting from fwd to reverse or vice-versa.