Even when the air inside your home has reached the desired temperature setting, you may still hear fan sounds or notice air coming from your registers and/or vents. This is normal and not typically the sign of a malfunction. The fresh air system sometimes operates on a different cycle than the HVAC system and it may cycle to keep fresh air circulating in your home and/or regulate the temperatures of the coils in your HVAC system. 

If your fan is running continuously and not cycling off and on, go to your thermostat and verify that your fan is switched to the AUTO setting.