When I had a similar thump mine got progressively worse, QUICKLY. I took it to the Chrysler dealer, and it turns out I had some severely degraded bushings in my rear suspension. This ended up bending one of my rear control arms. I thought it was all pretty strange as the car only had 40k on it at the time (just out of the warranty period naturally). The whole repair bill was close to $900. Keep an eye and ear on that thump... it could be something very serious.