When I moved to this village from London I quickly became aware of a constant low droning, as if from some motor or generator. It never goes off, and is most noticeable late at night, particularly when trying to get to sleep. I've established that it's not coming from anything in my house or those of my neighbours, and does not stop if all electricity in the area is turned off, say, when work is being done to overhead lines.
Though not hugely loud, it really got to me, and drove a wedge between me and my then girlfriend, who could not hear it. I've explored the surrounding area and it's definitely not an electricity sub-station, nor is it tinnitus, as has been suggested, as my then next-door-neighbour could hear it, too, even though other neighbours cannot. I even called someone out from the local council environmental health, but he could not hear it either! Ten years on, it has never stopped, though it does get quieter from time to time, and I suppose I'm now inured to it, much as how people living near a railway line learn to filter out the train noise.
I'm resigned to never finding out the cause.