I just bought my van over the weekend, so I haven't had a chance to investigate this noise myself yet. It sounds like a marble inside a sealed tin can being shaken back and forth rapidly. It's coming from somewhere in the rear, it's completely random and only does it in the first 15 minutes from a cold start. The PO told me it has done this from the time they bought it new and no one has ever been able to figure it out...Probably because it is so random, no tech wants to waste his time with it. The van doesn't have to be rolling, I heard it last night sitting still.
FWIW, I had a similar sound that went away after a big chunk of sheet metal fell from underneath the van. By the size and shape of it, it looks like it is some sort of heat shield for the cat or the muffler.