Terry S. wrote:
I bought my small one-story house in 1996. It is now about 26 years
old. There is no basement, but there is a crawl space under the
entire house that I store a lot of stuff in. The pipes for the house's
plumbing and the electrical and cable wires are all visible in the
crawl space.

Recently I started hearing loud clicks coming from under the floor in
the living room. I assumed they were coming from the crawl space, but
when I went in it I heard nothing. Now the clicks have turned into
what sounds like a noisy motor..

My oil furnace is in my garage and there's no sound coming from it.
I've gone back into the crawl space and checked it out from wall to
wall and still heard nothing, but when I go back into the living room
the sound is still there.

Any ideas as to what's going on?

Sounds can travel in odd ways through buildings depending on a variety of
resonant materals and spaces (and the combination of them). My suggestion
would be to listen to the house itself by putting your ear on a solid
material (like a wall or a floor), and follow the sound to the source as the
volume increases. Take a variety of readings (several dozen) in one area to
get a better idea of what's going on, and increase your data sample.

For more sensitivity, use a mechanic's stethoscope or an inverted drinking
glass.

My guess would be a forgotten sump pump that is buried somewhere under
there.

Jon