If you put the transmission in a different gear when the noise is occurring does the frequency change? If the noise occurs at 3000 rpm in drive, does it occur at 3000 rpm in second and low?
Trying to get to this:
Is it related to engine rpm, or transmission rpm?

Can you put the van on a lift and recreate the problem?
If so, a mechanic's stethascope might just be the ticket to isolating the source of the noise.

(on a side note- a very under utilized tool in my opinion.)

I suspect it might be torque converter / transmission.
How many miles on it?
Have you kept the fluid changed in the transmission?

edit to add: some reports of engine lift bracket bolts working loose and rattling.