I have an 07' 4X2 CrewMax 5.7L. I have had the same problem for more than ~2 years. It happens when I come to a full stop, usually rapid deceleration. I believe it is the transmission downshifting from 2nd to 1st. It doesn't happen all the time but several times a week is typical.
I am not to concerned about it. The truck has ~50k and I have plans to trade it in next year for a 3/4 ton. I am seriously waiting to see what the big three + Toyota' plans are for 2014 models.