I think it might be coming up thru the rock below your house.
UpNorth when we heard it, I was most obvious in the house and not outside. There it is ALL limestone. On the island we live on there is very little soil in fact...it is ALL ROCK.
Depending on where you live, I'd also get a radon test.
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All truths are easy to understand once they are discovered; the point is to discover them.-- Galileo Galilei
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Weezie looking at petunias 8.25.2017
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