Your toilet flushes just fine, but it doesn’t know when to quit. Perhaps it stops running and then starts up again suddenly or constantly leaks into the bowl. Either way, it’s wasting a lot of water and making that noise that keeps you up at night. Fortunately, it’s usually not difficult or expensive to repair a toilet if you know a bit about how they work. Be systematic about tracking down the problem. There are only so many things that can go wrong inside a toilet tank.