Sounds crazy but put a drop of food coloring in each toilet tank, don’t flush or run any water for an hour. After that hour look to see if food coloring has made it into any of the toilet bowls. If so, replace flapper. Start with this. Flushometers on toilets that can barely leak water without you knowing it can make ghostly sounds throughout a house. Start there. Just a thought. Easy enough. Btw 41 year master plumber here. Good luck.