What Causes the Engine Compartment to Make Noise After the Car is Turned Off?
2 Answers
Engine compartment noise after turning off the car is caused by the catalytic converter, uneven engine cooling, and turbo cooling. The specific reasons are as follows: Catalytic Converter Cooling Sound: The normal operating temperature of the catalytic converter is 400-800 degrees Celsius. After turning off the engine, its operation gradually stops, and the temperature decreases, causing the internal metal components to expand and contract due to heat, resulting in abnormal noises. Turbo Cooling Sound: Under normal conditions, the exhaust gas temperature reaches 600-700 degrees Celsius, accelerating the friction-generated heat from the turbo's rapid rotation. After turning off the engine, the heat expansion and contraction become very noticeable, leading to abnormal noises. Uneven Engine Cooling: After driving for a while, the engine temperature becomes very high. When the engine is turned off, the temperature of various components drops, and the heat expansion and contraction cannot occur uniformly. This inevitably causes friction between the internal metal components due to cooling contraction, resulting in abnormal noises.
Once when I was fixing a car, I encountered this situation—it's actually just normal thermal expansion and contraction. After turning off the engine, components like the exhaust pipe and turbo are still scorching hot. When the metal cools and contracts, it makes a clicking sound, similar to the principle of radiator noises in winter. This is even more noticeable in German cars—for example, a BMW's turbo cooling can keep making noise for several minutes. Another possibility is the delayed shutdown of the cooling fan, where it keeps running even after the dashboard has gone dark. This is especially common after driving on the highway in summer. As long as the sound isn’t harsh and there’s no burning smell, there’s no need to panic. What you really need to watch out for are continuous metal grinding sounds or hissing noises—those require immediate inspection.
