I suffer from this insanity too. I always had sensitive hearing and at night when things get quieter, I pick up the smallest noises. Are they real? Is it in my head? I've gone the route of turning individual things off and eventually cutting power to my house before to ensure it isn't electrically generated. In my case, I never found the cause. I believe that one was real since if I wear ear plugs I can't hear it any more. There have been others where ear plugs haven't helped.
You can try using a phone app to do a spectrum analysis of sounds it picks up. If the sound is narrowband it can show up clearly. Then you can move around and see where it gets louder. Doesn't work well for non-narrowband noise. You can try humming what you hear so you can compare frequency, but keep in mind that noises can have harmonics, so you may be hearing those and not the fundamental.
Worst case, if the noise remains present and annoys you, you can try masking it by playing another noise over it. I'm debating putting a PC in my bedroom at some point. Smooth fan noise is comforting to me.
