2005 Toyota Highlander Foul odor when just VENT is selected

I live in the high desert of New Mexico ( approx 6.5 " of rain all year so far) and while the air conditioner works fine there is a foul musty odor only when the Vent is selected. Doesn't seem to make any difference if outside air or interior recirculation air is on. Is there a filter that can be changed or some kind of disinfectant used? Many thanks Don