My toilet makes a hissing noise all the time. It's making my water bill sky high. What can I do to fix it?

It sounds like you have a bad flapper or the tank water level is too high.

Check the "critical level" Mark at least an inch over the rim of the white overflow tube.

Check the tank water level, it should be at the mark in the tank, 1" below the rim of the over flow tube when the tank is filled and the ballcock shut down.

Feel around the flapper flat edge. If it's wavy or you feel rough spots replace the flapper,
The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the ballcock and make sure it's connected to the white overflow tube and draining water in to the tube, open the link below will show you how to replace the flapper valve, good luck.

John


YouTube - Toilet Repair: Fill Valve & Flapper : Toilet Repair: Flapper