I have had my 12 SXT for about 3 weeks also,and have just started to notice the same `thump`.If you have the 32 gal. fuel tank,and the fuel level is between 1/2 & 3/4 ,this seems to be a common issue. It may be fuel sloshing that you are hearing/feeling.Others on this forum have said that they turn off their hill start assist,(from the EVIC) and the problem disappeared.Let the fuel level go below 1/2 and see if it improves.It may be caused by the lack of baffles in the tank.I am a Service Tech at a Chrysler dealership and have never seen baffles in any plastic fuel tank yet.Other than the occasional thump,I don`t see any cause for concern,and is not detrimental to the vehicle. Jeff