the hum , if the ear could see , would perhaps appear like a varicolored mist , expanding from all directions and of all sorts : from heart-monitors , ventilation systems , distant elevators racing ( or laboring ) up and downtheir doors opening , some smoothly , some with little squeaks , and closingto computer and other equipment fans , capped by the high-pitchedunhearable almostfrequency of various computer screens and like instruments .