It is most probably a vacuum leak. On most cars the heater controls are vacuum operated. If it is in the engine compartment it will be easy to find and fix. Start the car, let it run for a couple minutes, shut it off and listen for the leak. It will be a slight sucking sound. Replace or repair the hose. If it is in the dash it will be a big problem. You will have to take out a lot of your dash to find and fix it. The first place I would look is behind the heater controls. This is most probably where it is. If it's not there it could be anywhere in your dash and you will need a mechanic to fix it. It's gonna cost a lot if you need a mechanic. Good luck.