Toilet Flush Chain Is Sized Incorrectly or Damaged
The flapper at the bottom of the toilet tank needs to be fully shut after every flush to keep the next cycle of water in the tank. A chain is attached to the top of the flapper. The other end of the chain is attached to the extension arm of the toilet handle. A toilet makes a hissing sound when the chain prevents the flapper from closing or when the chain is so long that excess chain gets caught under the flapper. A chain that is too short will keep the flapper in a slightly raised position, allowing water to pass out of the tank—creating a toilet hissing sound.
How to Fix a Toilet Flush Chain
The toilet flush chain might just be twisted up and shortened or it may have been improperly sized when it was installed. In this case, the chain can be adjusted. If the chain is damaged, it can be replaced. A replacement toilet chain costs $2 to $3.
Flap Valve and Seal Are Dirty
The flap valve is a hinged flapper at the bottom of the toilet tank that lifts on command to permit water to flow into the towel bowl. The flap valve is blue, red, gray, or black, and it might be hard plastic or made of soft silicone or rubber. Over years of being immersed in water, the flap valve and the seal below it can build up slime, sediment, mold, and other debris. If one is dirty, usually both are dirty since they touch each other. This gunk allows traces of water to flow into the toilet bowl, creating toilet hissing sounds.
How to Repair a Dirty Flap Valve and Seal
Flap Valve and Seal Are Cracked or Deteriorated
The hinged flapper at the bottom of the toilet tank opens to let the water rush into the toilet bowl or closes to let the toilet tank fill up again. Because the flapper and the seal are made of rubber or silicone, these components are prone to cracking or otherwise deteriorating over time. Toilet hissing sounds are created by water slowly leaking through cracks in the components or between a damaged flapper and seal.
How to Replace a Cracked or Deteriorated Flap Valve
When the toilet flap valve and seal are damaged, they cannot be fixed. Instead, the toilet flapper and seal must be replaced. Use a universal 2-inch toilet flush valve repair kit, an item that costs $10 to $15. The kit usually includes a toilet flapper, seal, sealant, and sometimes a chain.
