That's just a coil or capacitor vibrating at high frequency. The switch mode power supply changes frequency depending on the load, and when your phone is fully charged the load is low so the normally-ultrasound drops into the audible range.

Nearly all chargers get like that eventually, I guess as the glue or capacitors start to fail.

The reason you can only hear it in some places is because it causes standing waves, and you move your head in and out of the nodes. Basically constructive and destructive interference.

Anyway it is almost certainly totally unrelated. It's always very quiet and doesn't cause any brain damage or any other symptoms.