It’s very possible. There’s another source of noise that I’ve noticed, it’s qualitatively different so I don’t think it’s related, but there is a life flight helicopter pad about 2 miles from me. Every now and then there will be a real low rumble in the house that you can actually feel more than hear. About a minute later the helicopter will fly overhead.

There is an auto assembly plant about 5 miles away, if it’s not in my head it’s almost certainly something from there. The ground here has a extremely high clay content, and when you get down a bit it’s like one solid mass. It seems ideal for transmitting sound long distances.

If you have enough time when you notice the sound, it might be interesting to turn off your house's main circuit breaker to see if the sound goes away. Might be a quick way to bisect the analysis.

Ah! Now that you mention it, I have done that. It didn’t make a difference and I think that is roughly when I started seriously considering this was all in my head.

I can see why that possibility would drive you nuts!

Then I second the idea of getting two recordings (baseline and when you hear this) using a suitable mic. If you can afford the hardware, it's got to be way cheaper than questioning your hearing and/or psyche for the rest of your life :)

Definitely worth trying! Thanks for the ideas and the support, 2020 isn't the best year for 'hearing weird things at home' haha :)

Glad to be a sounding board. Yeah, 2020 is definitely the year we could write the book, "Gaslighting Yourself for Dummies". Best of luck :)

Perhaps you could clamp a microphone to a metal rod stuck into the Earth to test that hypothesis.

Same for clamping onto the structure of your house for the vibration/fan one?

Finally, if all else fails, clamp a microphone to your head

I could hear a fait beeping in one corner of my kitchen, but if I moved ~3 feet in any direction it disappeared.

Eventually discovered that it was some kind of acoustical beat frequency from the motor noise in the refrigerator reflecting off the back wall. Moved the refrigerator ~1 inch further away from the wall and the sound disappeared (at least from the location I had been able to hear it)