>>311364564
>>311363468
>>311362037
>>311362313
I just tested myself and I can hear all the way up to 18kHz. Whether I can't hear beyond that or my speakers can't reproduce frequencies higher than that is up for debate.
I can also hear other similarly annoying stuff like some phone chargers just fine.

My Index base stations make no annoying sounds though. They have a whine but it's nowhere near 18kHz and so quiet that I can't hear it unless I'm right next to it.