2004 GMC sierra 1500 sle, slaping/thumping noise coming from bed under carriage can anyone help.? I have the same problem with my 2002 gmc and can not find out were the slapping is coming from. Took out my spare tire , took off my sway bar , replaced my shocks, took off tail gate ext. ext. , and i still can not find the noise.^&%^*^%&^((V(^V ^ I will check out the catalytic converter next. (anybody with any other ideas?)john