I remember something on HN about large cooling systems for data centres being responsible for some of these hum noises, but i can't find the reference.
My guess was cooling systems in general. I lived uphill from a residential complex and the units on the roof pointed directly at my house. The hum drove me nuts, but it was below any legal noise threshold. It was louder inside my house than outside.
Yes resonances are a fun thing. For very little vibration input, you could get exceptionally large acoustic output.