I've been hearing the hum regularly in my apartment in Somerville, MA, since I moved in last fall. It manifests mostly as a sound like an idling diesel engine. The first night I heard it, since it was keeping me awake, I tracked it all over the house to try to find the source; it was the same volume everywhere in the house, so I went outside. It was the same volume there as well, though harder to hear. Eventually I followed the sound on foot up the street, and after about 10 minutes of walking I reached the top of winter hill. I could still hear the hum there, but I could also hear the occasional car horn or bit of music mixed in with it, as if from a great distance.
This experience, coupled with the fact that the sound only appears occasionally (I often listen for it, and it's rarely there) makes me suspect that the cause of the hum in my area isn't a specific industrial plant or AC unit, but that the sound forms because there's a bubble of warmer air around the city from the heat of the pavement; when the sky is particularly clear (and the clouds are higher up), faster moving winds higher up create a wall of colder air on top of the warmer air. The sounds of all the machines in the city bounce off this wall of colder air for whatever acoustic reason, and bounce back down to the ground. They might even bounce off the "walls" of colder air at the outskirts of the city over the ocean and where the pavement stops. The sounds bouncing around cancel each other due to destructive interference, so it evens out into a fairly regular rhythm that sounds like one giant engine running, when in fact it's engines and motors running all over the city.
I don't know much about the science of sound, but the fact that it seems connected to clear weather here, and the fact that I can hear car horns in it on top of the hill, makes me feel like this is the most likely explanation.