Ha! I have both, worked for the telephone co. wearing headsets. But I also have fine hearing abilities. I agree with the waterleak, the pipes servicing the house can vibrate with small leaks producing loud but mostly inaudable tones. Sometimes you can feel them first before hearing them or identifying the source. A neighbors revolving roof vent took me a days to find.
But lets add this. I have an automatic water filler for my gas furnace, (an electromagnetic device) that was out of adjustment. My 'noise' was felt raher than heard and only lasted a short term @ times. It used to begin with a low deep thrum, caused by operating/opening the water shunt to fill the furnace tank (steam heat) and then rise in pitch to the inaudible range. I adjusted the water feed to the unit from 'full open' to 'closed and open 1/2 turn'. The shunt would shudder from the differances in pressure when used, lowering the rate of water flow available to the unit stopped the shudder and the noise. *Umm, I ignored it untill the shudder could be felt and heard loudly enough to merit attention, actually it scared the chit out of me and dissappeared -after I lept out of bed and ran to the basement. The at times only condition (how often it needed more water) made finding this one a challenge. Reducing the available water flow to the automatic unit immediatly solved the noise problems