You have air in the cooling system. The "bubling" you hear in the dash is air pockets circulating through the heater core. I'm not sure about your X, however a lot of new vehicles have an air bleed screw on the cooling system (usually on the thermostat housing or somewhere high on the engine) that you take out/loosen while filling the cooling system to purge the air. The reason they need those is because the hose drops below the highest point of the coolling system or the radiator fill neck is not the highest point of the cooling system. Once you get the air out of the system your heat will work fine. It is possible that you are losing coolant, howver it sounds like from what you are describing is that when the enghine cools it is purging some of the air and the fluid you are "losing" is displacing that air. This may be a symptom of another problem with the cooling system and not the problem as well (such as a leak allowing air into the cooling system or a coolant loss). I'd suggest taking it to a shop and have them pressure test the cooling system to see if you have a leak in it.