Could be a wheel bearing that is heading south, does the van make any
other noises like when your turning?  What if you are slowing and turning?

Have you flushed the tranny fluid regularly?

Ted

do you hear a thumping while breaking?>????

Is it not more a rattling/ticking noise like on some mountainbikes??.
In that case it´s nothing to worry about, the gearbox sounds like
that.
Most people never notice it, but when you go by a wall with your
window open  you can hear it clearly.I have a Voyager 1995 that has
320k kilometers on it(don´t know what that is i miles)that sound like
that.I was a little bit worried about that so i testdrived a 2001 at a
local dealer that has 5k Km on it and it sounds just like mine.

I also found this in the faq in this group.

4.   Transmission noise: when shifting/stopping, buzzng/ratcheting

Bob Meyer <robert_meyer@ncsu.edu> wrote about the Stratus (Cirrus/Breeze)'s
automatic transmission making a buzzing/ratcheting noise when shifting
gears or pulling to a stop. He said this noise, which also occurs on the
Acclaim and other cars, is normal and comes from the solenoids. It is most
noticeable from outside the car. He warned that a continuous buzz or whine
could indicate low fluid or a bad pump, "But if what you're hearing is only
during shifting from park into reverse or drive or coming to a stop, then
the dealer is probably telling you the truth."