If an air conditioner becomes dirty and is not regularly serviced, they can smell. Another reason is when the system is working on cooling the heat exchanger becomes moist. If the unit switches to heating the bacteria in the moist coil can smell and soon goes away.
To assist in removing smells, open your return air door and spray Glenn20 (or similar) for 1-2 seconds into the ductwork while the system is running. Always ensure your filter is clean and free from debris and grease, see Filter Cleaning.