BUZZFX4

So today I was riding as a pasanger in my truck (2004 crew) for the first time since it has gotten cold out. I noticed a relitively steady flow of cold air coming from under the front passanger side dash. The heat was off at the time so I turned it on and the flow stopped. Does any one know if there is a blender door that close of the system while in the off state other then the door that switches between in cab and out side air draw? The door that hands in cab draw and out side air draw seems to be working fine and has no obstructions.

Also, is it normal at full fan speed for the system to sounded like a 737 taking off with regards to air movement? Most of the nosie is under the dash.