[yash101]

It's nothing to worry about. It is a buck converter so what happens is that there is a transformer that is toggled on-and-off by a MOSFET, controlled by some very high-speed response chipset. That way, it can convert that high voltage to a lower voltage, higher amperage, with a theoretical 100% efficiency (though most are between 80-90%).

That sound is because when the MOSFET is toggled, the magnetic flux caues the transformer to resonate to nearby steel things/motors/magnets/earth's magnetic field.

So again, don't worry about it. Only worry about it if it is making a kinda low-pitched screaming sound, meaning that there's heavy load or it is shorted out. If you are getting that mean and groaning noise, pull out a multimeter and measure the amperage. If it is close to 5 amps, you've got yourself a problem!

Good luck and don't get scared by high-pitched sounds coming from these devices.