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.
Gus, I think we have the same noise. I was thinking it has to be something electrical since it doesn't matter if I'm rolling or stopped. So in looking at the wiring diagram, I see that there is a Evap. Canister Vent Valve on the undercarriage in the middle of the van...right about where the noise seems to come from. On my next day off, I'm going to run a jumper to it and activate it from inside the van to see if it makes the same noise. I'll let you know what I find.