Occult noise and the pulse of sanity

An undiscoverable hum has been haunting my second story listening room for two weeks.  I powered down AND unplugged separately my active speakers, then my preamp, then my DAC to no avail.  The hum persisted as I tracked what surely must be a dying transformer in some piece.  The sound very diffuse and steady.

Alas, the ceiling fan running just below the listening room was discovered to be the culprit.  Although a quality Minke fan, it nonetheless generated enough hum to transmit and make me crazy.  Hope this helps anyone else bird dogging this kind of noise transmitted through framing and flooring.