There is a hum on a close frequency as an air conditioner fan. It is and has been heard worldwide. I first learned of its existence a few years ago. I'm glad I've never heard it. It is a hum that doesn't go away, that would be extremely annoying.
The only thing I hear is more like a "whump" coming from the ventilation unit at the top of the YMHA directly across the street from my bedroom window. It is annoying as he!! and I can't get them to do anything about it. It isn't heard as much during the day when there are ambient street sounds, but late at night and in the early morning hours, my kitchen is the only place I don't hear it.
If you are close to an electricity sub-station you might hear a low hum from the transformers, if close enough and in quiet conditions, but that's all.
I wonder how real this supposedly ubiquitous hum really is, or if there is a certain self-fulfilling effect present.
If genuine it would be easy to detect and identify using a microphone and spectrum-analyser, and I don't know if anyone has tried it and if so, their results.
Thriftymaid 