Toilet-Working Intermittently and Hissing Noise-No Leaks

[lfsukhram]

Hi all. My house was built around 25 years ago, and I think the toilet might be that old. The toilet is making a hissing noise, but there is no water leak I can find and our water bill hasn't gone up. Now the toilet will randomly stop working for a few hours and then all the sudden the hissing noise gets really loud for a few seconds and then starts hissing normally. It will work when I hear the hissing noise. I am worried that there is an issue with the plumbing. We have a new toilet to install, but right now, cannot get anyone to come install it due to the coronavirus. Any ideas for how to fix it in the meantime or what it could be?

 

[Stigster]

The only thing that can make a hissing noise is either the cold water inlet valve filling normally or this same valve letting by when it should be closed.

If it is letting by the water must be going down through the internal overflow and into the pan. You don't have an external overflow there so that's the only place it can be going.

I can see in your picture that the water level within the cistern is currently below the overflow tube but judging by the water mark inside the cistern it has been overflowing. The cold inlet valve also has an overflow tube which is put into the cistern overflow tube. The hissing could also be the valve letting by and going down through there into the pan. It really does look like the water level has been rising above the overflow level though.

When it is next hissing, lift the lid and see if you can detect any water running down inside the pan. You will most likely need to replace that inlet valve on the left.

 

[moonlight]

You could just replace the seal on the bottom of the valve, if water is trickling into the pan. To test put some dry toilet paper on the back of the pan and watch.

 

[Stigster]

You could just replace the seal on the bottom of the valve, if water is trickling into the pan. To test put some dry toilet paper on the back of the pan and watch.

Do you mean the flush valve seal? I don't think the problem lies there as you can see the cistern has been overflowing by the tide mark within, plus the description of hissing from OP.