I'm looking for ideas on what might cause the very deep hum I hear in my house when it's quiet. It's been present 24/7 for many years. When I'm alone in the house and nothing is turned on it's actually loud enough to be annoying, but it's not loud enough to record on my phone.
I live in a semi-rural residential neighborhood with no industry within miles. I can turn off my main breaker and the noise still present, so I assume it's not coming from inside my house. I'm 2 to 3 miles from an Interstate highway. Most of the time when I'm outside I cannot hear road noise from the highway. We have county water but no natural gas or sewer. It's slightly louder in my basement. It's not louder near where the water pipe enters the house.