I had a similar noise in my 2002. My problem was actually the alternator and I suspect the same with yours. Mine was either a bearing or the clutch making the noise. The noise is always there but it is more pronounced when you're in gear, I don't know why. I just got a reman one from Advance Auto.
Put a hose up the the alternator and your ear to see if that's there the noise is coming from. If you can't hear it in park you will need a helper to put it in gear and hold the brake. Stand beside the van, not in front, of coure, to avoid any accident.
You can put the hose on the tranny as well to see if it is coming from there but I'm going with the alternator on this one. Changing an alternator is better than changing a tranny any day
Good luck and let us know the results.