Musty smell coming from A/C vents in car?

i have a 2005 Nissan Altima, i bought it 4 weeks ago and for the past week i have been smelling a musty smell coming from my a/c vents everytime i crank it. i have had the recirculation on for a good while, could that have something to do with it? would just running the a/c for a while fix it? anybody know what i need to do? thank you and God Bless