What Causes the Loud Noise When the Car Fan is Turned On?
3 Answers
When the car fan is turned on and produces abnormal noise, it is usually due to deformed fan blades, a damaged motor, or dirt and blockages in the air conditioning system. The fan automatically activates when the water temperature exceeds 95 degrees Celsius. If the fan blades are deformed, they will generate loud noise during rotation. Similarly, a damaged fan motor can cause abnormal fan speed, resulting in unusual sounds. Dirt and blockages in the air conditioning system can lead to excessive airflow noise. It is recommended to clean the air conditioning system. Typically, the cabin air filter should be replaced every 10,000 to 20,000 kilometers. If the environment is particularly dusty, the replacement interval should be shortened. Over time, the air conditioning system's pipes may develop mold or accumulate debris, so it is advisable to clean them every 30,000 to 40,000 kilometers. The car air conditioning system mainly consists of a refrigeration system, heating system, air distribution system, control system, and air purification system. The refrigeration system includes components such as the evaporator, air conditioning compressor, condenser, liquid storage tank, expansion valve, and high/low-pressure pipelines. The heating system primarily comprises a heater core, water valve, and blower motor. The system operates based on signals from various sensors that detect cabin temperature, evaporator temperature, engine coolant temperature, and other relevant switch inputs. These signals control the radiator fan, condenser fan, compressor clutch, blower motor, and air control motor to automatically regulate the cabin temperature.
I've been driving for over a decade and have encountered the issue of the AC fan making loud roaring noises several times. The most common cause is the fan blades getting deformed by small stones—even a slight bend can cause a thumping vibration when spinning. Another time, after washing the car, water got into the fan bearings and caused rust, resulting in a piercing metal grinding noise at startup. The most bizarre case was when I was fixing a friend's car and found half a plastic bag tangled in the blower—turning on the AC made it sound like a tractor. It's good practice to regularly inspect the engine bay, especially during seasons with lots of fallen leaves, and clear debris around the fan shroud. If the noise comes with a burnt smell, the motor is likely failing and needs replacement ASAP. Don’t let small issues turn into big problems.
A sudden buzzing noise from the fan is most likely a mechanical issue. Check if the bearing lacks lubrication, as dry friction during rotation can cause excessive noise. Sometimes, loose fan blades wobbling can also create strong vibrations, especially after high-speed driving when metal fatigue becomes more noticeable. Another possibility is debris like twigs or stones stuck under the AC evaporator, causing the blower to rotate with foreign objects, which inevitably creates loud noises. If the noise is only noticeable at the highest fan speed, it might be due to motor aging and insufficient power. These issues can be preliminarily checked by yourself—turn off the engine, open the hood, and manually rotate the fan blades to check for excessive resistance or any sticking sensation. If unsure, take it to a repair shop.
