> Supposedly the kokomo hum was tracked down to some industrial
> ventilators.

So I read. The UK site I posted claims, and appears to back it
up with survey evidence, that some hums are caused by or linked
to the national power grid. If really true, that poses quite a major
problem for the sufferers. Although the site includes a petition to
"stop it", I don't see anyone running to change the whole national
grid system. Not a minor undertaking

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