I bought a 2003 F-150 Lariat, 4.8L 4x2 about a week ago. Everything worked great at first, now I have this hissing/air coming from the dash. It happens while the truck is on, no matter if parked, driving, etc.. Ac/heater on or off, noise is still there. I have not yet taken the dash apart to see if i can pinpoint this noise and I am wondering if anyone has come across this. To better explain it is almost as if the air or heat is on and really low. Thanks in advance.

 

are all your heat settings working? Heater control's for the vents are vacuum operated

 

After your post I checked and in fact my floor setting does not work. It comes out the defrost when I do floor... every other setting works fine.