My iPhone charger emits a little very high pitch noise when my iPhone is charged... is it normal?
Have you ever wondered why the chargers are so small now days? Depending on your age, would remember that in old days the chargers were big and very heavy. It was the heaviest thing to carry around with your Laptop. Now days there are comparatively small an d light weight.

The new technology use high frequency transformers (15kHz range). That would be the Ultrasound range. If the circuit is not correct you would hear that sound.

Here is a sample how it sounds at 10kHz range.

Normally you should not hear the 15kHz, so in your chase the charger is creating lower (10kHz) harmonics, because it is not working properly.

Warning: If you have a dog make sure it does not go crazy.