I hear the hum as well sometimes, in different locations (my home office, separate from the house - here I can hear it during the day) or in the house, at night

I hear the hum as well sometimes, in different locations (my home office, separate from the house - here I can hear it during the day) or in the house, at night. When heard, if I move my head a little bit, the sounds goes away and usually comes back a second later. I thought this was a nearby engine/motor (there are a lot of houses with installed heat pumps in the neighborhood), but as I've read the Wikipedia article, my money is now on tinnitus. I've been running a FFY analyzer when I hear it and it's never been able to pick anything up actually airborne, nor any vibration through anything solid that would be picked up by a microphone.