Any motion that would make a soccer ball roll from one side of the truck to the other will make whatever it is, slide and clunk. As I wrote above, I can zig zag down the road and make it slide from side to side: very distinct slide/clunk, slide/clunk, slide/clunk. Is the fuel pump in the tank? And if so is it possible that it is sliding around in there? I am the only owner of this car and this is a new noise, but there has been from day one a clunk when you go over a bump--even a speed-bump. Could it be that an in-tank fuel pump has broken loose?