The noise you hear is coming from a component, called electromagnetic coil. Some coils are louder, the others quieter (almost silent). If you’re hearing the noise, then you’ve probably got the louder one.
The frequencies are around 13 - 14 kHz, and if you have a good hearing, this sound can be disturbing. It can be more noticeable during nighttime and in very quiet places.
 

To reduce the noise of the coil, we pour the resin to cover it. Sometimes during the process of adding the resin, micro bubbles of air are formed around the coil. When the resin cools down, the bubbles pop and can leave a tiny micro hole that causes the “sound leaking” and as a result, you can hear the noise. 

If the noise is really bothering you, we kindly ask you to open the ticket here. In the Subject please write: MD is noisy.