This situation is caused by the sound of the engine thermal expansion and contraction, which is completely normal and the car owner does not need to worry. When the engine is working, the temperature of each part is not the same. The temperature of the cylinder block generally does not exceed 100°C, while the temperature of the exhaust pipe will reach 400~500°C, and the temperature of the three-way catalytic converter is as high as 600°C. °C. Therefore, the expansion amounts of these parts are also different. After the car is turned off, there will still be some residual exhaust gas that will be discharged. When passing through the exhaust pipe, there is likely to be some movement due to the principle of thermal expansion and contraction. This is normal. We don't need to make a fuss.