Generally speaking, it is near the door or at the top of the door (old cars), and some new cars are installed on the rotating shaft. In any case, the electric swing door will rotate the rotating shaft through compressed air, or use the slide rail to push the door to slide, so as to open or close the door, because there must be compressed air in such a large car. The brake and clutch of the car are driven by compressed air, and the car is also equipped with an air compressor. In this way, it is also convenient to use compressed air for one-way transmission. Of course, the sound you hear may also be the sound of brake or clutch, because there will be the sound of compressed air release when using, especially when parking or shifting gears. However, for these doors, some cars use liquid instead of compressed air. Compressed air is much quieter, but the brakes also need compressed air. When the compressed air is full, it will brake. When it needs to be released, it will release compressed air. In the process of releasing into the atmosphere, there will be these sounds.

Buses usually have pneumatic swing doors. When the driver presses the switch, the solenoid valve is powered on, the air circuit is connected, the door pump intake and exhaust, and the door is opened and closed. Some can use the remote control to open and close the solenoid valve to control the opening and closing of the door