It sounds like GPU coil whine. But since it's adjacent to your AC power plug it's most likely the onboard charging circuit or power supply. I'm guessing the sound is an oscillating rectifier diode/bridge because one or more of the associated smoothing caps has failed.
The good news is that it would probably be a trivial repair at any qualified electronics shop, whether they replace the whole module or just solder out failed component. Much easier/cheaper than repair/replacement of mainboard VRMs or GPU VRMs. The bad news is that it'll probably get worse, noisier and noisier until diode breakdown finally surges power through a fuse and kills the whole power module.
I've heard much worse but this is still pretty bad and I think it should definitely qualify for RMA.
But you'll have to wait until someone official from @ASUS responds to this thread.
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