This problem is annoying. Once you flush the toilet, everything seems fine. Then after a few minutes, you hear the sound of water running in the toilet tank!
What's happening is that water has leaked out the bottom of the rubber flapper valve into the toilet and the water level in the tank has dropped.
Once it drops below the "fill" level, the toilet ballcock fills the tank back up.
To fix this problem I recommend this product from Fluidmaster® called the "Fluidmaster Flusher Fixer Kit." I've used this product and it's easy to install and works great.
To see learn how it's installed, see the tutorial Repairing a Leaky Toilet Tank Flush Valve.
