Who doesn't enjoy the calming sound of a box fan? I put the microphone right up to the motor to capture its hum as well as the background drone of the blades.
I hear simultaneous notes A#2 and C3. Probably the A# is the motor. Most motor hums in my house seem to be near A#, as evidenced here, here, and here. According to yet another article at Wikipedia that doesn't cite and references or sources,
Electric hum, mains hum, or power line hum is an audible oscillation at the frequency of the mains alternating current, which is usually 50 or 60 hertz depending on the local electric utility configuration.... Assuming a tempered scale with A=440Hz, a 60Hz tone is almost exactly halfway between A# and B two octaves below Middle C....So perhaps I'm hearing a 60 hertz tone in the electric hum of these motors.