We can put Hamburg (Germany) on the list of "humming cities" too. When I moved close to the harbour I started hearing a humming sound, espececially at night, almost exclusively indoors.
None of my flatmates heard that sound, so I started questioning my perception until I found another person living in the same building hearing that sound. Our bedrooms are located one below the other and the sound is most perceivable in that rooms, but we can hear that sound at other places inside the building too while others can not.
Some time later, articles in various local newspapers about that hum appeared (search for "Brummen Hamburg" if you're interested, unfortunately only articles in german), telling stories about various people along the river "Elbe" hearing that hum. I started reading about the Windsor and Bristol hum that time too.
A local politician started investigating that issue together with scientist from a technical university but - besides some theories (e.g. a big power plant) which couldn't be proven - with no success. The acousticians where cited that some of them could hear the hum at various places too, but in terms of sound level they couldn't measure it distincly from the "general" background noises of the city (with harbour, industry and much highway traffic).
In the last few years there wasn't much news about it but the hum is still there in varying intensity and duration.
Is a hum not more or less expected in the vicinity of busy harbors, even more so than in general industrial areas? Harbors attract a lot of possible sources that could cause a hum: maritime equipment, ship engines, generators, industrial equipment, large refrigerators (in containers or in warehouses), ...
You may want to read about the Kokomo, IN "hum" investigation which was traced back to an industrial source [1].
> Is a hum not more or less expected in the vicinity of busy harbors, even more so than in general industrial areas?
You're absolutely right, but there are different types of hums and many of them you can directly be linked to a source. Just to name the two loudest in my direct neighbourhood:
* Some kind of ships at slow speeds make very loud humming sounds. The loudest in my expirience are big car carriers (to be specific: Grimaldi carriers in Hamburg). The sound is very unsettling since it's at such a low frequency, that it's more "feeling" than "hearing". But the good thing is that it's over after the ship passes by.
* Some procedures in the nearby ship repair dock are very unsettling too. Especially sandblasting ship hulls is very annoying and for some reasons they mainly do it at night.
But the most distracting hum of which I was talking about in my parent comment has yet to be linked to a specific source. And it lasts much longer than my other examples, typically for days. So it has to be something which is produced only in specific conditions. This doesn't mean that this is ruling out all of your examples (e.g. large refrigerators could only be in use when the storage capacity is needed, it could be specific vessels etc.).
None of my flatmates heard that sound, so I started questioning my perception until I found another person living in the same building hearing that sound. Our bedrooms are located one below the other and the sound is most perceivable in that rooms, but we can hear that sound at other places inside the building too while others can not.
Some time later, articles in various local newspapers about that hum appeared (search for "Brummen Hamburg" if you're interested, unfortunately only articles in german), telling stories about various people along the river "Elbe" hearing that hum. I started reading about the Windsor and Bristol hum that time too.
A local politician started investigating that issue together with scientist from a technical university but - besides some theories (e.g. a big power plant) which couldn't be proven - with no success. The acousticians where cited that some of them could hear the hum at various places too, but in terms of sound level they couldn't measure it distincly from the "general" background noises of the city (with harbour, industry and much highway traffic).
In the last few years there wasn't much news about it but the hum is still there in varying intensity and duration.