Pontiac Grand Prix

I have a 2004 Pontiac Grand Prix GT2 with a 3.8L V6 and 21,000 miles. For a little over a year now when I am driving over bumpy parts of a road I hear a banging sound coming from underneath the car. I first made sure there was nothing in the trunk! I took the car to the dealership about 9 month ago and they could not find anything. They said that the shocks and exhaust seem fine. Then 4 months ago took it back again. They replaced the stabalizer link pins, cause being loose. The noise did go away for about a month. Since then it has been gradually getting worse again. Any advise? It is driving me crazy!
May 12, 2007.

Since the noise went away ater the Stab links were put in and it is now gradually comming back, I would re-check those links again. Maybe they weren't tight enough when installed.