Every few days to weeks, I hear a very deep humming noise. It's so deep that some older people might not even hear it, but just loud enough to annoy me and sometimes even cause headache. It often lasts for hours and it is audible at different times of the day (but basically never at night). I've gone around the neighbourhood to try to find the source, but couldn't really track it down (mostly because of traffic noise). It probably gets transmitted via the ground. This noise is pretty monotonous, but not just a single frequency (otherwise I could turn my head in a certain direction to no longer hear it).
How could I find the source of this sound? Just relying on my ears alone apparently isn't working. A cheap microphone might not even pick it up. I'm not completely opposed to buying/renting a specialised device, but would like to avoid that for now.
Update April: I've started taking more notes about the details of this humming.
It cleanly turns on and off, there is never any difference in pitch or volume, no variation, slow start or ending or anything like that. So it is almost definitely some kind of machine that causes this.
The hum usually plays for 1-5 hours, at various times of the day and week. Here is a table that shows some of the occurrences (but probably not all, since I sometimes wake up with it already playing, sometimes already listen to music with headphones and don't notice it, sometimes am simply not at home, etc.):
The top column is actually empty because I've never heard it after midnight. The next ones are empty because I'm rarely awake at those times. The others are somewhat complete.
The hum often plays for hours at a time, but often (especially after it had been permanently on for a while) it turns off for a few seconds to minutes, then on again for up to a minute, then off again and so on.
Update late May: The humming has not happened for over a month now. The last day in the table above was the last day when I heard it. That's nice for me, but it doesn't answer the question, so I'll not post that as an answer. I've also not marked any answer as accepted so far, because they're all either rather expensive or not applicable in many cases.
Another update on the same day: It's happening again. Yay…