1. Faulty Blend Door: The blend door is a component in your car's HVAC system that controls the flow of heated or cooled air into the cabin. If it becomes damaged or obstructed, it can create knocking or clicking noises when you turn on the heat.
2. Defective Heater Control Valve: The heater control valve regulates the flow of hot coolant from the engine into the heater core, which then warms the air that is blown into the car's interior. A faulty heater control valve could make knocking sounds if it's stuck or malfunctioning.
3. Loose or Broken Ductwork: The ducts that carry warm air from the heater core to the various vents inside the car can become loose or detach, causing knocking sounds. This can occur due to age, wear, or improper installation.
4. Expansion and Contraction: When the heat is turned on in your car, certain components may expand or contract due to temperature changes. In some cases, this can create knocking or ticking noises as these parts adjust. This is usually not a cause for concern unless the sounds persist or become louder over time.
5. Faulty Engine Components: In rare cases, a knocking sound when turning on the heat could indicate a problem with an engine component, such as a loose timing chain or worn connecting rod bearings. However, this is less likely to be the cause if the sound is only present when the heat is turned on.
If you're unsure of the source of the knocking sound when you turn on the heat in your car, it's best to have it inspected by a qualified mechanic to correctly identify and repair the issue.
