That is a phantom flush, and it means water is leaking from the tank into the bowl until the float drops far enough to trigger a refill. The cause is nearly always a worn flapper, and the part costs under ten dollars.
Confirm it in ten minutes with food colouring
Put a few drops of food colouring in the tank. Do not flush. Come back in ten to fifteen minutes and look in the bowl. Colour in the bowl means water is passing the flapper. That is the whole diagnosis.
Why flappers fail
The flapper is a rubber seal sitting on a plastic seat. Chlorine in municipal water, and especially in-tank cleaning tablets, degrade the rubber — it goes hard, or slightly warped, or develops a groove where it meets the seat. Three to five years is a normal life. In-tank bleach tablets can halve that, which is a good reason not to use them.
Replacement is genuinely simple: shut the supply, flush to empty, unhook the old flapper from the ears on the overflow tube, clip the new one on, reconnect the chain with a little slack, turn the water back on. Fifteen minutes. Take the old one to the store to match it, or buy a universal.
If a new flapper does not fix it
- Check the seat. Run a finger around the plastic rim the flapper sits on. Mineral build-up or a nick will hold the flapper off. Fine wet-and-dry paper, gently, usually sorts it.
- Check the chain. Too short and it holds the flapper fractionally open. Too long and it gets under the flapper. You want a slight sag with no slack pooling.
- Check the fill level. If water is running into the overflow tube, the fill valve is set too high or has failed. The waterline should sit about an inch below the top of the overflow.
Why it is worth doing this week
A phantom-flushing toilet wastes far more than it looks like. A leak small enough that you only hear the refill a few times an hour can still be several hundred gallons a week. It is one of the largest and most common invisible items on a residential water bill, and the fix is a ten-dollar part.
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