Assuming it's real and not an neurological artifact (heheh) it's possible your house has is resonating to some vibration and some of the energy is released as a higher/lower frequency noise you can hear. If it seems you can only hear it in one spot but can't get a direction on it the source might be an interference caused by reflection. We had a wood-frame waterbed sitting on a ground floor slab that would resonate to earthquake P-waves , if you were laying in it you would hear a distant bell tolling from a far-away earthquake. But you would hear nothing if not in the bed. It took a few near-by earthquakes to make the connection.
My dogs hear it... I have had one in bed and he would cant his head to one side when it started... one female will bark when it starts but they settle down after a few minutes. I do think it is using structures as a transducer.
LLS