I live in Waltham, MA and I began hearing a low frequency noise last March, so I used a tuner ap on my phone to detect the noise--that gave me an idea of the frequency and it told me I wasn't just going nuts. It bothered me for weeks. Then I moved to the other side of town. I still heard it a bit, but it went away sometime in April (If I remember right).

However, it came back in late October/early November, and I think it's gotten louder.

It's a low frequency, and not super loud--sounds like a distant engine idle kind-of, so it is easily masked by other sounds. This means I only hear it inside the house, or in a parked car (if the engine is off). I've detected it in Woburn as well as in Waltham, so I suspect it affects a fairly large area.