Now that the weather is getting colder I've turned the terperature control to "warm" in the car and moved the air control switch to come out at the floor setting. When I do this there is a horrible hissing noise until the car reaches normal operating temperature. The hissing only happens at the floor setting - if I move the air flow to the dash the hissing stops. This is EXTREMELY annoying! Any suggestions would be greatly appreciated.

My son once lost a snake in one of my cars, fortunately it was never seen again.

As for your vacuum leak, you need to either locate the source with your ears when it's leaking and repair/replace the part or have it vacuum checked with a vacuum guage to locate the source.

In your car, the little doors in the dash that direct air various places are vacuum actuated. As far as I can remember from the pictures in my Haynes, the servos are rubber things very much like the cruise control actuator you can see just behind your throttle cable drum under the hood. Sounds like the one that holds the door in the "floor" position has a leak that seals itself, or at least gets less severe, as it warms up.

These things are inside the dash and may be a real pain to get at.