I am a general handyman that was helping out an older couple fix some small items around their house. Leakign toilet, sink strainer, flood light, and landscaping.
They said they had a humming in the house and that they couldn't figure out the cause. They said it sounded like it was in the pipes.
My experience with these hums is due to toilets when they are getting close to being filled or something leaking. I looked all over the house and it didn't look like anything was leaking. The sound sounds like it is coming from upstairs, doesn't seem to be related to flushing the toilet or running the faucets. They can just be sitting there and all of a sudden hmmmm.
They said it has been like that for 5 years now and they are really tired of it.