Ford Expedition knocking sound from rear third row when rear air conditioning or heat is activated on Ford Expedition

Some Ford Expedition owners report experiencing a knocking sound from the rear third row when the rear air conditioning or heat is activated. This noise may be attributed to a faulty blend door or blend door actuator in the rear HVAC unit, which is responsible for regulating air temperature from the vents. Additionally, some owners find that the actuator itself can develop issues, such as broken plastic teeth, leading to the knocking sound during operation. Others have noted a thumping noise that resembles a ball dropping, which occurs when the rear vents are turned on and stops only when they are turned off. In certain instances, the knocking may be linked to an electric motor cycling, potentially related to the third-row seats, and can happen sporadically even when the vehicle is off. It is advisable for owners experiencing these issues to have the blend door, blend door actuator, and rear air system inspected by a qualified mechanic for proper diagnosis and resolution.