I don't know if this would be the same thing, but I had a thump when I would speed up or come to a stop in my 2000. It was fuel sloshing in the fuel tank. It would start to do it around 3/4 of a tank and this is the same general area you described.

I read a TSB on the thump and took it back under warrenty and they fixed it for me. Apparently they replace the whole tank, has something to do with the foam inside the tank.