Originally Posted by JimSmith

It is also possible your drains are plugged. There are some drain tubes that carry water from condensation or any other source that might make its way into the HVAC system, like rain water coming in with the air. I believe these drains are located on either side of the transmission under the car. Left plugged they will contribute to a musty smell developing in the A/C system. Try a search on the subject if you think you want to explore this further. The drains are discussed relatively frequently. Good luck, Jim