We all know that the majority of ordinary household sockets are 220V voltage (domestic), then the output voltage of the uSb socket is generally 5V DC, so to convert 220V AC to 5V DC output, then you must install transformer, rectifier and current limiting electronic components in the socket, and transformer and rectifier electronic components will have electromagnetic generation. Thus there will be a slight electromagnetic hum, this phenomenon is normal, there is no need to worry, the quality of the electronic original also makes this electromagnetic sound is very different, some light, some heavy, very normal.
The sound generated by it is due to ferromagnetic resonance, and the iron sheet produces magnetic vibration when it is energized, thus converting the high voltage to the low voltage working process.
Generally, the vibration of the USB charger with large current is larger, such as the sound heard by 2400mA will be much larger than that of 500mA.
If the sound control is smaller, it can be improved from the process and iron and copper wire materials, which will make the cost of USB chargers rise.