noise under dash in ventilation system

Hi! I've owned my accord for almost 2 years now. I bought it new. Great car, but about a year ago, I started hearing rattling sounds and noises that almost sounds like saying"psst"under the dash board, probably where the ventilation system is. I have the fully automatic climate control system and these noises occur on and off whether the system is on or off. Maybe because I'm a musician, my ears are sensitive to any sounds, but these sounds are becoming increasingly worse. Does anybody have any idea what this could be and how it can be fixed, hopefully without tearing the car apart? Thanks!