The more I think about this, the more I bet it's something electric. Either transformers or tranducers or condensers or something like that. Have you ever noticed how eerily quiet it gets when the power goes out? Just the constant hum of the refrigerator is surprisingly loud, but we tend to tune it out.
Next time you hear it, maybe you should try turning off your circuit breakers and see if it goes away (for those who've said it's louder in the house than outside).
Also, I'm not sure but I believe sound waves propagate faster and farther the lower the air density (S = elasticity / density?), because it takes less energy to push the wave forward, basically, so that might be a factor in the hum up here, too. Maybe even carries more when a low-pressure system passes over head?
Been a long time since I took a physics class, but it feels like it carries more up here when I hear those darn dogs barking at 3am.