Pavan is absolutely right here.
   Any mechanical oscillating system be it a string, a column of air, or whatever, generates a whole load of notes other than the fundamental. This set of notes is called the harmonic series, and consists of hole number multiples of the fundamental. For the note A-440Hz (vibrations per second): -
   Frequency    Interval ratio       Name
                expressed as within
                1 octave (2/1)

A   440Hz          1/1             Tonic
x2   880Hz          2/1             Octave 1
x3   1320Hz       3/2             Perfect 5th
x4   1760Hz       1/1             Octave 2
x5   2200Hz       5/4             Perfect Major 3rd (2nd Octave)
x6   2640Hz       3/2             Perfect 5th (2nd Octave)
x7   3080Hz       7/4             Harmonic 7th (2nd Octave)
x8   3520Hz       1/1             Octave 3
x9   3960Hz       9/8             Major 2nd
x10 4400Hz       5/4             Perfect Major 3rd (3rd Octave)
x11 4840Hz       11/8          Augmented 4th/Diminished 5th ?

    And so on theoretically fore ever, but in real systems the amount of energy in these "partials" diminishes as the harmonic number increases, so you only hear the first few.
   This series also produces other intervals between the harmonics.

   You can see above, the intervals produced by the GHB Drone correspond to notes in the Pipe scale. Its interesting to note that traditionally tuned Pipes have a Sharp 4th possibly the 11/8 interval from the series.

      All the best PP