This is due to the vibration of the transformer in the power adapter.
The vibration has been installed in peripheral power adapter, in order to convert alternating current into direct current, so when the adapter is used(or when bettery is charging ), the vibration will occur.
Generally, user may not notice this phenomenon when using the power adapter. However, the noise becomes noticeable when it is quiet.
Please understand that this phenomenon occurs on all devices, such as notebook, LCD or other peripheral devices that use a power adapter.
After the battery charging is stopped and battery is removed, the noise caused by the charging circuit, will disappear.
The battery itself does not create the noise and hence. is not the sound source.
When user disconnects the power adapter or the battery charging is complete the noise will disappear.