It is usually next to or on the top of the door (old cars). Some new door pump manufacturers install it on the shaft. Anyway, use compressed air to rotate the rotating shaft, or push the door to slide with the slide rail, so as to open or close the door. Because there must be compressed air in such a big car. The brake and clutch of the car are driven by compressed air, and there is an air compressor on the car. In this way, the use of compressed air is also very convenient for one-way transmission.

Of course, the sound you hear in the car may also be the sound of braking or clutch, because they also have the sound of compressed air release when they are in use, especially when parking or shifting gears.

However, some cars use liquid instead of compressed air, which is not quiet. But the brake also needs compressed air. The compressed air is charged to brake. When the compressed air needs to be released, there will be these sounds in the process of releasing it into the atmosphere. The bus is generally a pneumatic folding door. When the driver presses the switch, the solenoid valve is powered on, the air circuit is connected, the valve pump sucks in / exhausts, the valve is opened / closed, and the control valve is opened and closed.

I hope the above principle of the air pump can help you.