Banging noise under car!

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!