Sounds sort of like a leak but it is constant. My 77 did it too. Normal? My heater will run you out and the blower works fine so everything works. The vents open normally from floor to low to high to fog to ice.
does your Comet have AC??? If so it is a vacuum leak at the heat/AC controls or a vacuum line to the switches or heater/AC box under the dash.
My Comet HAD a/c but I removed it. All of it. So I should poke around in the dashboard for a leak...great. I am redoing my gauge pod tomorrow since the blue LEDs are stupid dim(but super cool). I'm going to paint the inside silver or white. Wish my needles lit up...anyway. While the pod is out I will feel around for the leak. It happens while the heat is running so I'll try to narrow it down to which setting does it.
