I have a 99 that at times air will not blow out of the vents. The fan is running becuase it sounds like a tornado in the truck when it does it, which is usually on smooth roads. If the road is rough or you beat on the dash it will work fine. When it stops working, if you change it to all out the dash vents it will start working fine, then return it to defrost or floor and it stops agian, have checked vacum and it is fine. Had a ford tech work on it and he says there isnt anyway any door could block it off completely, any ideas?