Environmental Sounds
It doesn't happen every time, but more often than not, when people are asked to hum a musical note in Europe they hum an A flat and people in the United States, Mexico, and Canada hum a B. The reason for this demonstrates how much environmental sounds are both a conscious and subconscious part of our lives.
The musical pitch A flat is in multiples of 51 Hz and the musical pitch B is in multiples of 61 Hz...almost identical to the 50 Hz AC line frequency in Europe and 60 Hz AC line frequency in North America.
People either consciously or subconsciously hear those pitches in lights, AC motors, and in the hum that's often amplified in audio signals. So, just like an annoying song that gets stuck in our heads, these pitches become "earworms". Then when we hum a note with no other reference, most people will hum that pitch.