have a look here i have the same problem intermittently..
http://986forum.com/forums/general-discussions/56844-strange-noise-dash-area.html
it's the foam getting caught in the flaps. it does it with the AC off as well because those flaps adjusts through which vents the air comes out of.
i still get it once in a while but it doesnt bother me.. anymore..
also, i know that if i open it up to clean it it's just a matter of time until more gets caught in the flaps. eventually the foam moves on and everything is well again
