For reference, here's a thread from VW Vortex that started me down this path, even though it's not the exact same behavior:
http://forums.vwvortex.com/showthread.php?5640499-2002-Eurovan-A-c-vents-roar-when-pressing-the-gasAnd a specific post from that thread:
"Just took care of this on our 2000 MV....
For some reason, when you step on the gas when the climate control is on "AUTO", the climate computer shifts into recirculate mode. The vacuum motor that controls the recirculation flap does not work on my van. When the computer tries to activate this valve, it leaks. That doesn't allow sufficient vacuum to keep the flap to the center dash vents open, hence the loss of air flow and extra noise.
This vacuum motor (part # 7D1820625) is in the far right hand side of the air distribution housing that contains the fresh air fan, etc., and is accessible from the passenger side. There is a removable panel under the passenger air bag, that allows you to reach up to the vacuum motor.
I haven't pulled the vacuum motor yet - it is held by two screws in a really tight spot. Until I get a chance, I pulled the vacuum hose (Black with green stripe) off the vacuum valve rail (N53 in the Bentley - bright red box) and put a plugged piece of hose on the rail. I don't have a recirculate mode, but I also don't have my wife asking me what the heck is wrong with the air conditioning.
As I said above, this only happened when the climatronic was in Auto mode, or the recirculate feature was turned on. Can anyone explain why the computer would want to recirculate airflow when accelerating?"
This morning I also spoke to the shop that does most of the maintenance on our van, and based on the behavior I reported and his own research, he is leaning towards the controls of the recirculation flap as well.