Same thing happened to me about 3 days ago. It is probably the gasket that goes around the cabin air filter bracket (Plastic piece held with two bolts)and the car body around the opening. Take the filter out, then unbolt the plastic piece that the filter snaps into. Look behind it and you will probably see what was once a pretty good gasket. I used foam weatherstripping about 1/2 inch thick all around it and it hasen't leaked since.
