F150PitBull

I've got an '05 Ford F150 4.6L V8 SuperCrew XLT 2WD. When I'm at a stop, truck is running (idling) in D (Drive), and I let off the brakes, truck starts to roll like every vehicle with an automatic trans does, just as soon as I push the gas and the truck starts to accelerate, there is about a half of a second delay before I feel, and barely hear, a clunk under the truck, almost like something is sliding back then hits something and stops. I was told by one person it may be a loose trans mount. Now accepting any and all suggestions and ideas, and any of either are much appreciated. Thanks guys (and gals).