There are cases where there is a buzzing noise when connected to AC adapter (including when charging). This is caused by the vibration of the transformer within the AC adapter.
All VAIO PCs
This is caused by the vibration of the transformer within the AC adapter. There is a transformer built in the AC adapter. Converting AC current to DC current causes it to vibrate when connected. It may not be bothersome when you are using it normally, but the vibration can be troublesome when using in a quiet environment. This sound will not only come from the AC adapter, but it also resonates from any type of electronics that are connected to AC adapter. With notebook PCs, the sound resonates from both the AC adapter and the PC’s charging circuit. If the battery is removed because it will no longer charge, the sound will stop. The battery does not create the sound, so normally the sound would not come from the battery.
When the AC adapter is unplugged, or when the battery is fully charged, the sound will stop.