3rd Gen Rushing sound

Hi all--

First off, thanks to everyone who takes the time to answer questions here :)

I've just noticed a loud rushing sound coming from (or at least, from that
direction) underneath the passenger dash in my FD. It lasts about 3-5
seconds and sounds like some sort of liquid or particulant is being kicked
up and circulated about. This happens when revving from idle--after 5
seconds or so it stops--if I let the engine idle for about 5 seconds then
rev the engine again it's back. I can't hear the sound from the engine
compartment over the engine noise, but inside the car it's pretty loud.

Any ideas...?

--Paul