two things.

well, okay, three....

Need more information on the year and such...
Additionally, is their air coming out of the vents, is it clean smelling or musty?

If you have something that is equipped with one of those cabin air filters, it could be a problem.
If you have a traditional air intake system for your cabin, you may want to pull the heater core out and make sure it is not clogged in the inside, as well as the outside. May have a bunch of dust, environmental goo ro whatever blocking the flow.
Lastly, you say the coolant is full. Are you referring to the over flow bottle (plastic, semi-clear thing) or the radiator itself? The plastic bottle is useless as far as telling how much is in the radiator. Try pulling the radiator cap off, starting the engine and keep an eye on the level while it is warming up and when it has reached mormal operating temperature. If it drops more than 1/2 inch below the bottom of the neck, you aren't running on a full system.

Okay, let us know so we can get a better idea as to whats really bothering you.