Ok, so shutting my house down, and using a handheld radio, I still get the noise.
Weird thing: It appears to be coming from every power line, cable line, and phone line on my house and the neighbor's. I can hear it when I place the radio close to the power/cable boxes on both my house, and the neighbor's.
So, what's the next step to hunt this down? Start walking the street, and doing the same on the poles?