Why is there a current sound when Xiaomi Smart Air Purifier 4 is turned off?

Solution:

This sound is the current sound produced by the inductor on the DC-DC circuit. Since the motherboard needs to convert the 24V voltage into 5V or 3.3V voltage, it needs to constantly switch the inductor at high frequency, and the inductor has been converting electrical energy into a magnetic field energy, and then convert the magnetic field energy into electrical energy. This energy conversion will cause the inductor to generate a slight current sound, which is a normal phenomenon.