fan still running after heating/ac airflow stops
 

When I turn the fan (heating/AC) off, the airflow stops, but I hear an additional fan running. Is this normal or isthere anothervent that I can shut off to stop the fan from running? If it is normal, what's the purpose? In the owners manual, it refers to the 'lower vent', but I am not clear on what they are referring to or how to operate it. any help would be appreciated. thanks -john