> wife and I are sleeping, she will occasionally be awakened by a
> "shaking." We live in northern Illinois, in a rural area, miles away
> from any urban development, gravel pits, or anything such like. I
> consistently sleep through it.

Sure it is not a large truck on a highway or a train? Depending on the
ground layers such a heavy transport vehicle could shake a reasonable area.

Another possibility could be subsonic vibration from a railway loco exhaust,
i.e. not actually the ground shaking. I have experienced this when a diesel
engine went past with the motor just idling - it must have been using very
little fuel at the time, and mostly pumping air through the cylinders.

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