The fill valve has a leak in it or if it has been replaced the setting is 2 high.if the valve is old - it needs 2 b replaced.sometimes the shaft or wire that i used 2 set the level, corrodes off.
Turn off the valve & tie up the ballcock...
Infiltration
You should not ever hear your tank filling unless you are using hot water somewhere. Your tank is under pressure always so it would never be filling if you were not using hot water.
The fill valve (ball valve UK) is siezed or clogged. As the water level rises, the float-ball rises and closes the valve when the water reaches the proper level.
Stop filling him/her up with air.
Open the supply valve and also stop it from being air bound
Stop filling it up.
The float valve is faulty, it does not shut upon filling of the cistern.
Because something's wrong with it. Exactly what is hard to tell, and the wording of the question doesn't help at all. If it's filling up but not shutting off (it continues to run even after the tank is full, with water going into the overflow drain), the problem is most likely that the shutoff valve needs to be adjusted. If the water continues to run but the tank isn't filling up (and there's no obvious puddle of water around it indicating a leak), the flapper valve probably isn't seating properly (this is the classic "jiggle the handle" problem). If the water isn't running at all, it's probably shut off at the wall valve (or somewhere further up the line).
I've read that it was a shock to put them in colder water when they were just in warm water. All you have to do is simply replace the water with the same temperature as the original water.
Either the flapper is letting water through or you need to replace your flush valve or there is something stuck between the stop and the flush valve.
gettin the gas in