The working frequency of the internal components of the power adaptor is within the audible frequency range of human ears (20 Hz to 20 KHz). People with sensitive ears may hear this noise. This can be avoided by unplugging the adaptor when not in use. Each adaptor has undergone rigorous safety certification and testing. The noise is generated by the normal operation of the product and does not affect product use or pose any safety risks.
Don't hesitate to contact us if you have any further questions.