hvac

[cavx5]

hi everyone...i have 2005 x5 4.4 sport/prem package with 17k..just noticed a weird noise coming from dash, drivers side, but cant pin exact location. sometimes it sounds like it is coming from underneath the steering wheel and sometimes it sounds like it is behind the instrument cluster maybe both...it sounds like a vent opening is trying to close or open..like when you change the vent direction from feet to face it is almost like an exhaling noise...you hear it approximately every 20-30 seconds and the noise lasts approx. 1 second..that is the best way i can describe the noise...any help...thanks rob