Well, this is interesting. I was going to respond: I hear it even when I'm outdoors so it can't be transformers.

However, then I decided to check if my phone could hear it, so I fired up my DB Meter app. And funnily enough, there is a little spike right around 12kHz, which seems to be right around where my tone is. It's not a huge spike, but it's definitely there.

Weird.