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.
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Polymer section voltage is about 3.7-4.2V, you need to use the DC-DC inverter circuit to boost to 5V from the USB interface output 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, in theory is a normal phenomenon
This situation is not normal, the general mobile power supply will not make a sound.
The reasons for this may be:
1, there may be water vapor on the mobile power motherboard, resulting in a slight short circuit of the motherboard.
2, the mobile power motherboard in the process of boosting the sound, but the sound is usually very small, almost inaudible.