How to find source of deep humming?

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 row 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…

This doesn't really answer how one could methodically find the source of something like this (so I won't mark it as accepted), but I did happen to find the most likely source a few hours ago: A Diesel engine of an old, non-electric freight train, running in standby.
I was driving my bike over a bridge that one of these trains was standing under, so I was able to hear it much louder (ouch, instant headache) and clearer, together with the rest of the frequency spectrum of its sound, which normally got drowned out by street noise whenever I was trying to find the sound near where I live.
This definitely explains why I thought that it might have come from the vague direction of the train station and it also confirms my suspicion that it was some kind of machine (because the sound only ever turns on or off cleanly, no gradual change in volume or pitch).

The only thing I can do now, apart from moving, is to contact the company that owns these trains and ask whether these train engines could be turned off completely when the train is not moving. That would also be better for the climate. I bet they have some kind of reason to run it in standby, so I don't estimate my chances to be that high, but it's worth a try.

Update: Apparently it was not the Diesel train. I found out about the source literally the exact moment when it stopped mattering: While I was saying goodbye to a neighbour after already having finished loading the moving truck, they told me that it was a highway. All other frequencies were just muffled by the house. No idea why I only noticed it so rarely and randomly, maybe it had to reach a certain level of traffic to be perceptible.