Is it a rattle or a clunk? If it's the clunk - check out the following link. I've had this done twice on my '02 since new. Once under warranty and once myself about 9 months ago (still clunk-free at this point).
http://gm-trucks.com/home/content/view/232/25
Hope this helps.
http://gm-trucks.com/home/content/view/232/25
Hope this helps.