: We have a 2-story home, and when the downstairs bathroom sink or toilet is used, we hear a noise
: from the pipes in the house that sounds like a foghorn
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Most likely the foghorn sound is from the fill valve on the toilet or a faucet washer on the lav.
I would lift off the lid of the toilet tank and observe the flush and refill process. Some toilets have a leaking "flapper" that slowly drains the tank down low enough to cause the fill valve on the toilet to refill. If the refill valve (ballcock) has a bad washer, it can set up foghorn like vibrations in the pipes.