I was just reading about hummadruz on the Northern Earth website and I think I may have the answer. The info about the phenomenon is here:
And here's my (possible) solution:
Probably caused by a person developing a freak sensitivity to their own
nervous system. Hearing the low hum which comes from the electricity in
the human nervous system vibrating below the lowest notes on a piano.
The nervous system's electrical activity vibrates in the range of 0.5 Hz
up to frequencies sometimes in excess of 33 Hz. The lowest musical note
on an 88 key piano keyboard is 27.5 Hz (lowest 'A' note -
sub-contra-octave) which sounds very low and dark and
mysterious but in terms of the human nervous system that note is a very
high rate of human electricity. We only get up to that sort of frequency
during hyperactivity or anxiety. These frequencies are called "High
Beta". So the rest of our electrical activity, when we aren't
hyperactive or anxious is in the range of the ELF (Extremely Low
Frequency). For instance, one octave below the lowest note on the 88-key
piano would be a note of 'A' vibrating at about 13.75 Hz, which is
smack dab on the borderline between Alpha and Beta waves. Alpha is when
the nervous system is dreaming or meditating, Beta is normal, everyday
consciousness which focusses on work and ordinary things. A chromatic
musical scale of 13 notes from a note of 'A' at that borderline of
Alpha/Beta extends upward, culminating in a top 'A' which is the lowest
key on the piano.
Human hearing is able to hear down as low as about 16 hz, which is
within the Beta range. So we cannot actually hear sound a low as Alpha
waves, but we can sense them as rhythmic feelings.
This information is helpful in terms of sensing within a room or a space
of some kind. Also helpful with understanding the use of hymns in
church or the Sanskrit 'OM'. Nervous system states vibrating at rates
lower than Alpha level are into Theta, which is about 4 to 7 hz and is
the level of Shamanic trances, deep meditation and sleep.