2017 MKC 2.3T. After the vehicle is turned off, there's a faint electric motor/fan humming sound. It seems to be coming from under the left rear passenger seat/rear compartment. It stops after 5-10 minutes. Then, when any door is opened, the sound starts again. This occurs when no power has been applied, ignition or auxiliary. Just opening a door it starts after about five seconds. What could it be and is it abnormal?