What are the potential causes of a musty smell coming from the furnace, and how can it be resolved?

The potential causes of a musty smell coming from the furnace could be mold or mildew growth in the ductwork or on the furnace filter. To resolve this issue, you can try cleaning or replacing the filter, inspecting and cleaning the ductwork, and ensuring proper ventilation in the area around the furnace. If the smell persists, it is recommended to contact a professional HVAC technician for further inspection and maintenance.