You can. The transformers inside electrical appliances and mains adaptors create a vibrating magnetic field, and many people can 'hear' or feel this.
If anyone wants a more technical explanation...
and if you don't, look away now 
Electricity comes into our homes at 230V (assuming you're in the UK) and many electrical appliances need much less power/voltage than this to work (about 19V for this laptop, for instance). The mains power therefore needs to be 'stepped down', and our homes are full of transformers that do this.
A transformer is essentially two coils of wire wound around an iron core, and it works by creating a magnetic field, which creates an energy flow from one coil to the other. The power is stepped down if there are more loops in the 'in' coil than the 'out' one (or up, at power stations for instance, if there are more loops in the 'out' coil).
The magnetic field vibrates at 100Hz (twice the frequency of UK alternating current), and if the transformer has or is next to anything at all that moves - which they mostly do/are (the circuit board it's attached to, or its plastic casing, for instance) then that/those will vibrate at 100Hz too.
The human hearing range is approx. 20-20,000Hz, so the 'mains hum' is at the very bottom of this - but many people can hear it, and it drives some people crazy! (It used to keep me awake at night when I was younger, but I think I'm going a bit deaf now!)
You can check whether it's mains hum or tinnitus you're hearing, Unacceptable, by unplugging some stuff and seeing if the noise stops! :)