Anthony:
Drive it along side, right up next to a building slowly (like the back of
your Wal-Mart or in the rear of a strip mall).
Do so with each side of the van next to the wall.  This will accentuate the
sounds that you are hearing.  My guess is that it is your CV joints. The
noise only while moving is obviously not engine related. The decrease in
noise on revving the engine points to the CVs.  It would seem that your
mechanic would have checked them when your rebuild was done; but stranger
things have happened.
HTH
--
Stephen
Chillicothe OH