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).