It is almost always the flapper, the ten dollar rubber seal at the bottom of the tank, and you can confirm it in fifteen minutes with a few drops of food coloring. If the color seeps into the bowl without a flush, the flapper is done. The fix is a genuine do-it-yourself job.
The fifteen-minute diagnosis
Take the tank lid off and put five or six drops of food coloring in the tank water. Do not flush. Come back in fifteen minutes. If the bowl water has color in it, water is leaking past the flapper — the rubber disc at the bottom of the tank — and that is your whole problem.
If the toilet runs in bursts every few minutes, that is the same thing: the tank slowly loses water past the flapper until the fill valve wakes up and tops it off. Plumbers call it a phantom flush.
The ten dollar fix
Turn off the shut-off valve behind the toilet, flush to empty the tank, unhook the old flapper from the two pegs and the chain, and put the new one on. Take the old flapper to the store with you — matching the style matters more than the brand. Leave a little slack in the chain; a chain that is too short holds the flapper open and recreates the exact problem you just fixed.
If the dye test comes back clean
Then listen. A constant hiss, or water trickling into the overflow tube in the middle of the tank, points at the fill valve instead — either the float is set too high or the valve itself is worn out. Adjusting the float is free. A new fill valve is about twenty dollars and installs with the same shut-off-and-empty routine as the flapper.
When to worry
A running toilet can quietly waste a couple hundred gallons a day — people usually find out from the water bill, not the sound. If you have swapped the flapper and the fill valve and it still runs, or the toilet is old enough that parts no longer match anything on the shelf, that is the point where a repair call costs less than another month of guessing.