I have never heard the low hum, but I have tinnitus, and I hear a high-pitched ringing. Recently I learned that some researchers think tinnitus is produced by the brain itself. It is trying to reconstitute high-pitched frequencies lost by 'insults' to the cochlea.

If you asked me if I hear my tinnitus everywhere and all the time, I couldn't tell you. Usually life is too interesting for me to think about it.

Maybe those who hear the hum have lost low frequencies, and the brain is trying to replace them. And perhaps the reason they don't hear it everywhere has to do with distractions, blood pressure or allergies.