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It's possible that the sound is caused due to interference on the USB connection; test by disconnecting only the USB cable from your computer. If that makes the noise go away, try this as well -- with the USB cable connected, disconnect the power supply from the laptop. If the noise goes away, the problem is your computer's charging circuit is not properly isolated from the USB chipset.
In either case, you may try using a ferrite core on the USB cable to reduce the amount of interference.