Intermittent hissing noise from Toto fill valve even after replacements

[Mevans42]

The fill valve on my Toto Vespin II was failing. I have no DIY skills, so I had my plumber replace the valve. While he was here, he swapped out the old flapper and replaced it with a generic brand. Afterward, I was still hearing an intermittent hissing noise. The plumber thought that the flapper might be a bad fit, so he replaced it with an OEM flapper. There was no hissing noise while he was here testing everything, but it reappeared an hour later and continues to occur intermittently. The plumber concluded that the new fill valve must be defective, so he replaced it again. Despite repeatedly changing out all the relevant parts, the hissing noise is still occurring intermittently. I believe that the water in the bowl is also very gradually refreshing itself. I'm completely baffled and would greatly appreciate any advice on what to do next.

I captured the hissing noise in this video, which also shows some slight rippling in the tank water.

 

[Jadnashua]

Shut off the water inlet to the toilet and let it sit, overnight would be ideal, but at least a few hours. Prior to that, use a pencil or something to mark the water level inside of the tank. My guess is that it is dropping, and the noise you hear is the fill valve just doing its job refilling the tank. If it doesn't drop too much, the valve doesn't open fully, and that tends to be a bit noisier than when fully opened like when you flush.

If it is losing water, there's a few possibilities:
- the flapper valve is either defective or mis-adjusted (the biggest error is too much slack in the chain and a link sometimes gets underneath, or too tight, and it won't let the flapper down fully)
- there's a crack in the overflow tube and water drains out to the point where it is whole
- the hose is not clipped above the top of the overflow and when pushed down into the overflow, it can siphon, causing the fill valve to turn on to refil
- the tank is leaking around where the flapper valve seat is attached to the tank. If that's it, taking the tank off and tightening the big lock nut on the bottom should stop it...worst case, it needs a new gasket
- There's some crud on the seat of the flapper valve assembly...if you rub your finger around that, it should be smooth. If it's not, the flapper valve won't be able to make a good seal.
- if the tank were hissing constantly, it would either be a big leak from below, or failure to shut off, and water is just running down the overflow and being replaced. That should have been obvious, and wouldn't be intermittent. There is normally around 1" or so of the overflow tube sticking above the water level.

 

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[Mevans42]

Thanks so much for your comprehensive advice! I will do what you suggest, and I'll post an update once the issue has been sorted out.

 

[Terry]

I've been using the Korky 3060 3" flappers lately for those.