The laptop power cable consists of two cables with a transformer in the middle. The transformer is making a tiny noise while plugged in. I think it's normal... is it normal? What is it exactly?
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.
A hum is normal. If you hear a crackling or sparking noise, the transformer may be about to die, and may constitute a fire hazard.
The hum comes about because the transformer works by running the alternating wall current through some loops of wire to make magnetic fields, and running the fields through other loops of wire to reconstitute the electric fields at a different voltage..
A side effect of this is a set of periodically alternating mechanical stresses on the core of the transformer. Stresses mean motion, and alternating motion in the right band means audible vibration in the air.
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?