After re-reading this thread, this is what comes to mind.

I remember in Watership Down, and Fern Tree Gully there was mention of a noise or hum in the distance. The sound of machines in those cases.

I also remember something in the book Doctor Zhivago by Boris Pasternak about a hum but it was more ethereal.

I must go see if I can find it.

Edited to add: Can't find it right now. Will post when I find it as it is quite superb.

I have fairly sensitive hearing. I hear noises that others don't. My son who is on autism spectrum was troubled terribly when he was younger by certain sounds that none of us could hear.