Perhaps at a different time, I'd have written this off as BS, but I am quite interested to know what it is, considering I am in Southern CA, in the desert, (where sound travels extremely well as it's all flat) and I am hearing it too.

I am confident that I can tell the diff between planes, nearby traffic, and power line hum. None of these sounds are what I am hearing though, or at least I don't believe they are. It is a rather eerie sound though, especially since it is very audible once you go outside, and it is nearly impossible to get an idea of where it's originating from, all I know is seems like it's from above.