>> It could be coming from the fuse board, and be due to a ground
>> fault condition, causing ground currents from neighbouring
>> consumers to come down the neutral and then to
>
>I thought of perhaps buried cables, could be power or data, or
>even someone in the area (she's not completely remote) who may
>leave a faulty monitor on at night

The reason I mentioned this one was an experience my parents had, where the
power pole supplying them and a neighbour was mounted on a bank outside the
houses, and as the bank slumped the neighbours power cable stretched, and in
due course the neutral broke, but the phase was still connected. My father
had been wondering why his distribution box made all sorts of buzzing
noises, and presumably a heap of other neighbours had the same problem,
probably without realising it, as this neighbours return current made its
way through the multiple earth returns back to the power supply.

Eventually the pole leant far enough for the phase to break (with a shower
of sparks, I remember seeing it happen), and after the power board repaired
the connection the noises from our distribution box went away.

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