I have had this issue since shortly after I got my Crossfire. It would come and go. Recently, the noises were very loud , very often and very scary and only while stopped at with my foot on the brake. If I put the car in neutral, it would not happen. I don't understand the correlation, but that is what stopped it. I had my Crossfire in the shop lately to get the ignition replaced ($$$) and the brake switch replaced. While it was there, I figured I needed an oil change. (I was overdue.) So I got that done also. When I got the car back, I noticed it has not made the thumping sound for 3 weeks. (Knock on wood!) It had been making the noise while stopped at red lights for the past 2 or 3 months, almost every light. The only thing I can figure is the oil change may have done the trick, or maybe it will start happening again soon.