I've had issues with this but I determined that it was some kind of tinnitus that occurs only when it's very quiet. It was right at 60 Hz too.
Definitely not ruling that out yet. I've had two people claiming they can hear it after I prompted but that might be power of suggestion.
Would explain many things, like hearing it far away from home and having a hard time to nail a consistent recording.
One young woman, who spent the night here some weeks ago, prompted me the next morning at breakfast. That made my day, because I was almost convinced of an auditory hallucination at that point.
Years ago I chased some low-frequency noise, which might have been the house just resonating with some frequencies from traffic and other things. I spent hours trying to eliminate it (a fence outside, supports inside). I ended up just having a bathroom ventilation fan with a weight bolted to one blade to make it badly off-balance, screwed to the baseboard so it would resonate the wall some to create its own low-frequency sound to mask the other at night. These days I just have an old small subwoofer connected to an mp3 player playing brown noise with strong low-frequency content, at a low volume. It knocks out all the cars idling and driving by, especially in the morning.
I was actually relieved to find that what I was experiencing was likely internal (tinnitus), since it meant I didn't need to go on another chase trying to find a case/solution. Well, as long as it only occurred in dead silence, since even a quiet fan made it go away. If it was all the time I think I'd go out of my mind. It's actually not acted up in weeks, even in dead silence (which also confirms that it's not external, because I couldn't have many days in a row of dead silence all day in the house even though they were busy days out in the world).