Could it be that the sound is more coming from the firewall area, near the passengers side? You should find that the blower motor is located there, probably behind the glove box (not 100% sure with the Caravan, but it is typically where they are located on most vehicles), and it is probably the blower motor starting to fail. Often when the wear out, it is the bearing or bushing on the motor shaft that wears, which allows the shaft to vibrate back and forth some, and that is what causes the humming sound. Get a new blower motor, and that should fix you right up.
