You’re almost asleep when you hear it – the sound of the toilet tank filling. You know no one’s using the toilet, so why does it sound like it’s flushing?
Most likely, you’re hearing the toilet tank refill with water because there’s a leak at the flush valve. Water flows from the tank into the bowl, then down the trap into the sewer line. This happens slowly, and you don’t hear a thing as the leak continues.
What you do hear is the tank refilling as if the toilet had been flushed. When enough water leaks from the tank, the float lowers, triggering the tank to refill, which sounds the same as when the toilet is flushed.
To check what’s happening, put some liquid food coloring in the tank. If there is a leak, colored water will appear in the bowl. The fix may be as simple as wiping debris from the mating surfaces of the flush valve or replacing the flush valve.