A toilet that keeps running after the tank should have stopped filling almost always comes down to one of two parts: the flapper, a rubber seal at the bottom of the tank that’s supposed to close and hold water, or the fill valve, which controls how the tank refills and shuts off. A flapper that’s worn, warped, or sitting crooked lets water seep past it constantly, which the fill valve then tries to replace, and the two fighting each other is what you hear as a toilet that never quite goes silent.
It’s a small part, usually inexpensive, and it can waste a genuinely large amount of water over a month if it’s left alone. The harder question is often not which part failed but why it failed early, since Weston’s water carries enough mineral content to shorten the working life of rubber seals faster than in some other regions.