I have a 05 Z71 Offroad Crewcab with 113000 miles. There is a "knocking" sound coming from somewhere under the front of the truck (Its not engine noice) Its something physically and audibly knocking back and forth go over bumps at low speed. I can acually make it do it by slightly swerving back and forth at low speeds. I can feel it in the floor board. It seems to be worse when its cold. My Dad has a 03 GMC Sierra that seems to be making the same noise. I've checked everything I can think to check and its driving me nuts!

 

Check the bushings in the stabilizer bar. I have seen worn out bushings make a noise like that. I would also check the control arm bushings.