2000 Lincoln LS odd mell form heater when car is idle

Recently my car has been encountering some issues with the heating system. When set to vent and 90 degrees it blows hot air as long as the car is in motion, as soon as I come to a stop(stop sign, red light ect.) It starts blowing cold and vents a real bad oder like something is burning. Also the other day as I was stationary at a red light I saw some smoke bellow out from under the hood but the car did not say that it was overheating. Cars getting high mileage and I want to prevent serious breakage with preventable matinence if possible.