1993 Mercury Sable FLOOR HEATER BLOWS OUT OF DEFROST VENTS

I have checked for vacuum leaks under the hood; but can't find any. I do notice a "hissing" sound coming from the heater vents around the drivers' side area of the car. Under the hood on the drivers' side near the firewall area, I can hear the same "hissing" but can't find any leaking vacuums. It almost sounds as if the sound is coming from the inside of the dashboard. Can anyone point me in the right direction as to what is the problem; how I can get to it and what needs to be done. The heat from the heat/air vents work fine and the ac from the vents work fine. The only problem is the heat from the "floor" and "mix" control blows out of the defrost vents. Thank you for your help.