Well, a plugged drain hose is a possibility, but usually when that happens, there is enough slop-over that the floor mat on the passenger side of the car becomes damp/soggy.


Next time the car is on a lift (or you are crawling around underneath), take a coat hangar (or other device), locate the drain hose, and check for obstructions - do not look up (lol)

1999 sc2 with the same noise
 

I just bought a used 1999 sc2 with the noise you described behind the dash. The car seems to run perfect and I did not notice the noice until I had the car for a day or two. It seems to happen only after the car has been sitting for a while. If I am out running around it does not seem to happen between trips in the car. I know the thread if a little old, but if you found out anything new, I'd appreciate hearing it.