I have on occasion been hearing 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 some others have heard, 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 and try to hear if it is coming from outside. I have thrown the main circuit breaker to the house to shut down all electric appliances and it still 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!
I recall many years ago when I still lived with my parents 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 it occurs most nights if I 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 that it's a form of tinnitus, which I do suffer from a bit, but I know it is 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). The 'Hum' that I experience is about 95Hz. Not everyone can hear it. Some people are more sensitive to low frequencies than others and I do know that I have an intolerance of deep bass.
When a kid drives past me with their sub woofers 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 remember once actually being sick at a live music event because of the deep bass. I also remember where I used to work there was some earth works being carried out nearby that were producing incredibly deep vibrations and I seemed to be the only person that noticed. 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.
Some interesting links:
World Hum Map