Have you checked the level of coolant in the radiator?
If it has dropped, replenish it and then check it again in a day or two.
Is there an oily film on the inside of any of the windows?
If your coolant level drops, and if you have any oily residue on the inside of the windows, then it is fairly evident that you have a leaking heater core.
There is also a possibility that there is a small leak or seepage under the hood, and that the smell is being pulled into the cab when you run the HVAC system. Have you checked for the telltale signs of a coolant leak under the hood?
If there is no leak under the hood, then it would appear that you have a bad heater core. Start saving your $$ for this possibility, as the labor costs can be very high for this repair job. If it is a leaking heater core, don't defer the repair for very long, as aerosolized coolant is a very nasty and toxic substance to breathe.