If you can hear the cistern topping itself up between flushes, that small sound is the most expensive thing on this page.
Hard water wears the parts out
The mineral content here stiffens flappers and fill valves faster than softer water would, so they stop sealing properly. It is an inexpensive part and a short visit, and worth sorting as soon as you hear it.
Older homes, older flanges
Houses from the forties and fifties often have a cast iron flange under the toilet that has corroded away. Fitting a new toilet on top of that without dealing with it just brings the same problem back, so we always check while we are there.
If the toilet rocks
Any movement means the seal underneath has failed, and water is going into the floor rather than down the drain. In older homes with wooden subfloors that leads to rot, so it is worth dealing with rather than living with the wobble.
Repair or replace
Parts are inexpensive and most toilets are worth fixing. If yours is original to the house, needing attention regularly and using far more water per flush than a modern one, replacing usually pays for itself.