Cars air ventilation blows air when off?

My cars ac/heater always blows air even when the ac/heater is completly off. it isnt a strong hit of air only a light blow that happens as i drive. if i come to a stop itll stop blowing but as i drive off again itll begin. is that normal? if not any idead what it could be? also when i turn the ac or heater on it gives off a weird smell sorta like mildew or dirty. any ideas would be appreciated. thanks in advance