Yep.. I hear it too.. but I also play in a band and I was thinking it's either due to my old age
or that ringing you get in your ears after a concert...it's that kind of high pitch. I can't remember when it started but it's been with me for a few months at least.
Some nights where I live I hear a really low frequency hum too, as if coming from a ship out at sea.. a sort of low engine noise. But it's not every night... maybe that's connected to the wind direction?
Would be interesting to know if these frequencies are being broadcast.
One thing to note though, the higher the frequency, the shorter the distance it can travel before attenuating. So to transmit this, you would need a LOT of transmitters. Cell phone basestations would be able to do this. - but their antennae are set for Radio Frequencies... 500Mhz and upwards... The antenna would not be suitable for broadcasting an audio signal of 20KHz