Why is the laptop charger thing (transformer) making a noise? [duplicate]

"It's a transformer.

Not Optimus Prime, but a boring device that steps down mains voltage to the voltage your laptop requires.

A slight hum is normal."

"Laptop chargers these days might be switching-mode power supplies with a transformer which operates at a much higher frequency than the 50/60Hz mains, because transformers work better at higher frequencies, and can be made smaller and lighter.

Is the noise high or low pitched?"