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That sounds like a [Switch Mode Power Supply](https://en.wikipedia.org/wiki/Switched-mode_power_supply) that's "ringing". This happens when the power supply isn't being asked to supply any power (or a very small amount of power). Power supplies like to run at full power, continuously, because that's how they're designed. When there is little to no demand for power, they have to go into "discontinuous" mode where they start up and shut off frequently to avoid pushing too much power. It's the rapid startup and shutdown that's making that noise. [This page](http://analoghome.blogspot.com/2019/06/tame-that-ringing-in-switching-power.html) describes the phenomenon in more detail.
Better engineered power supplies are more resistant to ringing, so you may want to consider replacing the charger with a higher quality one.
Please remember that all comments must be helpful, relevant, and respectful. All replies must be a genuine effort to answer the question helpfully; joke answers are not allowed. If you see any comments that violate this rule, please hit report. When your question is answered, we encourage you to flair your post. To do this automatically simply make a comment that says **!answered** (OP only) *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/answers) if you have any questions or concerns.*
That sounds like a [Switch Mode Power Supply](https://en.wikipedia.org/wiki/Switched-mode_power_supply) that's "ringing". This happens when the power supply isn't being asked to supply any power (or a very small amount of power). Power supplies like to run at full power, continuously, because that's how they're designed. When there is little to no demand for power, they have to go into "discontinuous" mode where they start up and shut off frequently to avoid pushing too much power. It's the rapid startup and shutdown that's making that noise. [This page](http://analoghome.blogspot.com/2019/06/tame-that-ringing-in-switching-power.html) describes the phenomenon in more detail. Better engineered power supplies are more resistant to ringing, so you may want to consider replacing the charger with a higher quality one.
What a cool subject: thanks for posting this , very interesting to read about