Loud Thunk

I get a loud thunk after i go over a bump. The problem is that it is not on every bump, infact i think it is actually usually after i go over a hole. And it seems to be delayed after i go over the bump. I looked underneath and bounced the car around, but i cant make it make the noise while its sitting still or see anything underneath or wrong with the shocks. Any ideas?
Oh the van has a load leaveling system.

one more update today when on smooth ground i heard the thunk again and this time i felt the back of the car move up. I had not gone over a bump or anything. I guess it is a shock

do you have the original rear shocks?
I replaced my load leveling shocks, and it wound up that the bolt hole had a larger diameter that the original. The result was the same as you describe. Basically, the shock would bang on the bolt that runs through the base of it.

glad I traded that van