2000 hissing under dash when heat/ac off

Just bought a 2000 SLS. Has a hissing vacuum noise when heat / ac is in off position only. pulsates. door switcher seems to work. Dash tear-down? mixer?
:shakehead thanx

Could very well be a vacuum leak. There are at least 3 vacuum operated blend doors under the dash. Does the vehicle have Auto climate control or manual heat/ac?