The sound of the charging treasure is emitted by the energy storage inductor, which is relatively slight, and it is normal to listen carefully to hear the words in a quiet environment.
Polymer section voltage is about 3.7-4.2V, you need to use the DC-DC inverter circuit to boost to 5V to output from the USB interface to charge the mobile phone, boost the voltage needs to first let the direct current into high frequency pulse, and then use the inductor or transformer boost, if the pulse frequency is within the auditory range, there will be noise, theoretically it is a normal phenomenon. There are two reasons for this:
① Mobile phones support fast charging, the required current is too large.
② The resistance of the charging cable is too large. You are advised to use another cable or a 2.1A port.
It means that the connection has been successfully charged