I wouldn't lose sleep over this, except the unexpected and fierce sound startles my girlfriend and cat every time.
So I just replaced the refill valve on our toilet (using a kit, so everything from the lock nut to the overflow spout, but not the plunger or handle). It works great. No signs of leaking (at least not on the floor or walls). It does a full refill every time right up to the refill line printed on the back of the tank. The overflow spout is above, not inside, the overflow pipe.
Every so often (maybe once an hour), the toilet shrieks for a half-second, like the float drops just enough to cut on the flow and then rises with just a squirt of water.
My first thought was evaporation, but 1) I'm surprised it's that sensitive and 2) I know our old fill valve didn't do this (at least I don't remember it doing it, and I'm sure both gf and cat would have mentioned it if it did).
The one thing I didn't tweak was the water level screw, because the critical point line and overflow pipe were the specified 1 inch apart (eyeballed) and the water fills to exactly the above-mentioned line.
If it is the water level, is this spontaneous and short burst indicative of needing to lower or raise the water level? And can someone explain why it happens at all? I imagine the water-level adjustment changes where the float "floats" so if the water level is where it should be after a refill, why would it run 30-60 minutes later for a short burst?
Best Answer
The most common problem that causes the symptoms you describe is a slow leak around the flapper. A very small amount of water drains into the bowl over time, lowers the tank level a slight amount and activates the float valve of a couple of seconds to refill the tank. What you should do before getting carried away dissecting the fill valve, is to check the flapper, clean it and it's seat well and be sure it is centered on the drain. If there are any signs of wear or cracks around the edges, replace it. Monitor the tank level between phantom fills, does it change at all? The second thing is to slightly lower the float so the water fills to apx 1/2 inch below the fill mark. This will eliminate the possibility that you may be losing water over the overflow, and tripping the fill valve.
These checks are simple, cheap and often solve a very common problem. Good Luck.