I have on occasion heard a strange noise that occurs between midnight and sunrise. It is low in frequency with a slightly varying amplitude and sounds like a lorry engine running in the distance, not idling like others have reported, but as if the lorry is stationary in neutral with a foot lightly on the accelerator, or when a lorry is struggling up a hill with a heavy load. This is no lorry though because it goes on for hours at a time and only at night. Maybe it's only noticeable at night because everything is quiet and still. I first become aware of it in the bedroom, but once noticed it can then be heard around the house. The sound seems to stop if I open a door or window to try and hear if it is coming from outside. I have thrown the main circuit breaker to the house to shut down all electric appliances and still it persists. I have pressed my ear to the adjoining wall of my neighbour in case there is a fridge or something on the other side, but I hear nothing. This is a very interesting mystery!
Many years ago when I still lived with my parents, I recall hearing a similar sound and wondering what it was. That was about 3 miles from my current location, the Medway Towns in Kent, UK. Back then I only heard it a couple of times but now I notice it more often, especially if I can't sleep and am awake into the early hours. I've researched the phenomenon and found that others around the world have had the same experience, although there seems to be places where it's more common, the UK being one of them. Some think it's a form of tinnitus, which I do suffer from, but I know it's not the same. Using a tone generator, I have worked out that my tinnitus has a main frequency of about 8Khz (there are other frequencies of lower amplitude mixed in there as well) but the 'Hum' that I experience is about 95Hz.
I Know I have an intolerance of deep bass. When someone drives past me with a sub woofer booming, even for a few seconds, I instantly get a headache and may even feel physically sick, which can last for some time afterwards. I was once actually sick at a live music event because of the deep bass! Where I used to work, I remember some earth works being carried out nearby that were producing incredibly deep vibrations which only I seemed to notice. It was so bad that I had to get away from it whilst my colleagues carried on blissfully unaware. I found it unbelievable that they couldn't hear it, although I guess it was more of a feeling than a sound. Perhaps a perception. I guess some people are just more sensitive to it than others. Or maybe it is just a form of tinitus!
Some interesting links:
World Hum Map