What you are most likely hearing are switched-mode power supplies, the cheaper the nosier.
https://en.wikipedia.org/wiki/Switched-mode_power_supply
"The operating frequency of an unloaded SMPS is sometimes in the audible human range, and may sound subjectively quite loud for people whose hearing is very sensitive to the relevant frequency range."
Even analog power supplies will generate a 50/60hz hum (depending on what the power-lines are providing)