Too many possibilities. Ball joint, sway bar bushing or link, control arm bushing, shock absorber, tie rod end to name a few. I recommend you have a thorough suspension/steering inspection as this could be indicating a safety issue.
Hi Precision Automotive Repair, I have the same problem. My Chevy dealership first told me it was my Tie Rod, paid to have that fix its not that. Now they are telling me its the Ball Joint and Bushings and that even if I do repair this the noise will not stop. The price tag on this $1,392.00 Bushings and alignment.
Another possibilty is the intermediate steering shaft. In the early 2000 model years of GM vehicles, this was/is a big problem. Not sure about the Express van, but the trucks all had this problem, all the way past 2006.