...a faulty valve within an HVAC (or heat pump) unit.
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We have a guest bathroom that is used fewer than 40 times a year. A few years ago, after I had finished cleaning the tub, I heard a dog howling. Sounded like a Basset hound. I looked outside fully expecting to see a dog in the back yard and very close to the house, but there was none.
The husband was out working in the yard. I went out and asked him if he had heard the dog. He had not.
After I went back upstairs, I realized, upon hearing the “dog” again, that the sound was actually emanating from a pipe in the shower. So strange. I suppose some air had gotten caught in the line. Have never heard it again.