* Faulty water pump. A faulty water pump can also create a humming sound. This is because the water pump is responsible for circulating coolant throughout the engine, and when it fails, it can cause the engine to overheat.
* Faulty fan motor. The fan motor is responsible for blowing air through the heater core, and when it fails, it can create a humming sound.
* Loose belts. Loose belts can also create a humming sound. This is because the belts are responsible for driving the water pump and fan motor, and when they are loose, they can slip and create a humming noise.
* Debris in the heater core. Debris in the heater core can also create a humming sound. This is because the debris can restrict the flow of air through the heater core, which can create a humming noise.
