You are likely hearing the worldwide "hum" that thousands of other people hear. It sounds somewhat like (my description) a distant diesel generator droning away at a low-medium frequency bolted to a cement slab, maybe a mile away.
It has been rather loud for the past several months at our place and at one end of the house (where the house supports are attached to the bedrock. Underneath the house (it's a pole home) it is louder but hard to hear except at night or early in the morning. Through the day, if there's no wind, I can hear it at certain parts of the house and outside. My partner cannot hear it, but wants to. I've told her once you've heard it, you can't unhear it and trust me, you don't want to hear it.
It has nothing to do with power- you can shut it off to check.
It is not local traffic/industry/planes/pumps as is there are none.
You can only hear it in a dead quiet environment. It's way below the noise floor of even my quietest interface and mic combination so you can't record it. All you do is boost noisefloor hiss and it masks the hum. Averaging won't find it.