When you changed the fluid what did you replace the PS fluid with?  The book
says ATF (Auto Tranny Fluid, Dexron) and not that brown PS fluid for other
makes of cars.  To properly bleed the system the book says to get both front
wheels off the ground and to turn the steering column from the right as far as
it will go to the left as far as it will go a few times (with the van not
running) and top off the fluid, then start the van and shut it off quickly
several times and top off the fluid, then start the van and turn the wheel
back and forth again a few times while you keep checking the level and topping
off when neccesary.  That is straight from the Bentley (excuse my paraphrase).
The noise you hear could be cavitating of the PS pump which is caused by air
in the system.
Or is your belt loose?
Ken Wilford
Van-Again
John 3:16