MacBook Pro w/ Retina Display 15-inch 2012 making strange cricket-like noise (only when plugged in)

Hi guys, I have a year old retina MBP. Recently, it just started making this high-pitched whirring cricket-like sounds. Although pretty quiet, the sound is pretty annoying. This sound has a few strange characteristics that make it hard for me to pin it down to any component:

- the sound only occurs when laptop is PLUGGED IN, when my laptop is not plugged (on battery), there is no sound

- reducing the screen brightness, reduces the sound but does not make it go away (even with no backlight, the sound is still there)

- the sound occurs mostly when performing going into MISSION CONTROL or swiping between desktops. It also sometimes happens randomly, but that is less common. It follows the inputs of trackpad and keyboard, though mostly mission control swipes and desktop changes. It doesn't coincide with intensive CPU, GPU, RAM or SSD tasks (ie running a CPU/GPU heavy app, or compressing a file does not increase the quantity of sound)

- I am not very sure where the sound originates from, might be from the top left of the keyboard (q key), but it is also louder near the speakers.

ALSO:

- It is not from the fans, as they occur when fans are running slow or fast and this sound is unaffected by any changes in fan speed

- reducing backlight has no effect

- turning automatic graphic off or on has no effect

- resetting NVRAM, etc. has no effect

Most likely theory:

- some kind of faulty capacitor maybe?

- but then which part of the laptop to replace....

- I don't have Applecare, but I do have a third-party warranty; they've agreed to replace the faulty part...

- I've read the forums and have seen people with similar problems (though never this exactly; replacing fans and logic board does not seem to help)