1. Blend door actuator: The blend door actuator is responsible for controlling the flow of air from the heater core and air conditioning unit to the vents in the dash. If the actuator is damaged or faulty, it can cause a knocking noise when it is trying to move the blend door.
2. Heater core: The heater core is a small radiator located in the heater box that transfers heat from the engine coolant to the air flowing through the heater box. If the heater core is damaged or clogged, it can restrict the flow of air and cause a knocking noise.
3. Blower motor: The blower motor is responsible for circulating the air through the heater box. If the blower motor is damaged or faulty, it can cause a knocking noise when it is trying to operate.
4. Debris in the heater box: If there is debris, such as leaves or sticks, in the heater box, it can cause a knocking noise when the blower motor is trying to circulate the air.
