Every plug-in power supply ("wall wart") I have ever used starts off silent, but eventually starts to emit high frequency switching noise. It usually happens only when the charger is on, but not charging anything. I presume it is due to the capacitors failing, or maybe the glue around an inductor.
My question is, why do they all use switching frequencies around 18 kHz which is audible and really annoying? Why not go up to 25 kHz or even higher?