We had a "hum" sound in cambridge MA. Its one of those sounds when you walk through you don't think anything of it, but if you live near it.... (I was one of the lucky as I couldn't hear it from my bedroom). Incredibly hard to track down, it was determined to be Fans on top of a building. And it was fixed.

The sound bounces around so much, just walking around I couldn't pinpoint it on a couple nights of trying. Its wierd to hear it, but not be able to determine direction. The city figured it out.