Laptop makes audible high frequency sound when AC adaptor is plugged in?

It seems when I'm using my laptop on just battery, the noise is neutralised, also it seems to go away with the charger and battery are in constant use.

when I use the laptop purely on AC power, it makes this high frequency sound only really noticeable when you put your ear to the machine or near the fan.

is this down to something serious or normal and why does it happen?