That happened to me one afternoon about two years ago when it was
uncharacteristically quiet in the house. There was one spot in my kitchen in
which I kept hearing a long, drawn-out, one-note sound. It almost sounded
like a motor running quietly, but not quite. After much investigation, I
discovered it was the wind blowing the attic vent on the roof and it was
making the noise as it was spinning. It probably always does it, but I
didn't notice until it was totally quiet in here, which isn't often, believe
me.