GM-GMC: 2001 Silverado HVAC, hvac system, fan work

Question
Here is a 3 part question:
1. The HVAC system in my truck is making an unusual sound.  When the selector switch is selected to blow air through the front vents a loud flapping sound starts.  When the selector switch is selected to any other position (ex. to the floor, front vent/floor, defrost) the flapping noise stops.  What is making that flapping noise?
2. Fan seems to be working too hard and not blowing enough air.  What can I do to make the fan work better?

3. The A/C only blows cold air when the selector switch is placed all the way to the cold setting (all the way counter clockwise).  If it is moved away from this position, it will not blow any more cold air.  It's like the A/C turns off and the fan just blows air with no conditioning.  What do I do with that?

Answer
It sounds to me like you have something obstructing your ducts. See if you can pinpoint the area the noise is coming from and try to get access to the duct.  I would bet something is blocking the doors from moving or a door has come loose.  You may gain access by removing the blower motor which is on the passenger side under the glove box.
Hope this helps.