I woke up last night hearing an odd high pitched noise that I am not used to. I usually keep a few fans running because I have a train that runs nearby and the white noise blocks it out. I tried methodically turning off all the devices in my room and turning them back on independently to identify the source of the noise. The result: All the fans in my apartment were emitting the noise at the same time.
The devices in question are: 1 electric fan, 1 air purifier (which includes a fan), the ceiling fan in my bathroom, and (possibly) the fan inside my air conditioner (I will need to reverify this when the problem recurs).
I have tried unplugging these devices and moved them into separate rooms with different outlets and verified that they still produce the sound.
Even stranger, at some point in the night the sound stopped, which means that all the devices simultaneously stopped making this noise. The sound has recurred intermittently since then.
My thinking is that this has something to do with an electromagnetic field affecting the motors, though some of my coworkers suggested it could relate to a power surge.
Is there anything else that might explain this? Is there an experiment I can use to verify what the source of the problem is? And finally, how might I fix the problem (without turning them all off, as then I would be able to hear the trains again)?