One of the toilets is making a running water noise.

First, a little background. About 4 or 5 months ago, this toilet would suddenly, about every 2 minutes, activate the water fill valve, make noise and top off the tank. A new flapper valve fixed this issue just fine.

Now, beginning about 2 weeks ago, I started just barely hearing the smallest sound of water running. It's gotten progressively a bit louder. I never hear the water valve activation that I heard with the previous problem. A listen with a mechanics stethoscope confirmed it coming from the tank, and turning the 1/4 turn shutoff valve off stopped the sound, and it came back when the 1/4 shutoff valve is turned back on.

I put dye in the tank, none migrates to the bowl, so I assume the flapper valve is ok. No external leak can be found. I'm making an uninformed assumption that it may be going down the overflow tube.

I'm not anywhere near being even a plumbers helper, and I'm stumped.

Anyone know what the cause, and the cure, would be?

Thanks in advance.

I think your uninformed assumption is spot on, and a new ballcock is the answer.

Adjust the float level down about 1/4 inch. The too high level is letting water go down the overflow.

The water level should be well below the overflow tube. A couple inches, usually. Dyed water that goes down the overflow tube will enter the bowl.

I've seen pinholes and longitudinal cracking in both metal and plastic overflow tubes.

How high is the water? Older toilets have a line embossed into the ceramic tank back interior. Some overflow tubes have a line on them.

If the water level is below the tube weir (a term I've just made up), sounds like your ballcock is passing water, as you've noted. Fluidmaster 400A is my preferred unit. If water is tight to the tippy-top of the overflow, lower the water level.

Consider this a consolidation answer, as both responders said the same thing.

And take what we call "a hard look" at the overflow tube.

Yep.

Another way to verify very slightly seeping ballcock is to...

1. Flush the john
2. Wait till it refills and shuts off
3. Make a pencil mark on the inside of the tank exactly at the water level
4. Check it in an hour or two to see if it has creeped up above the mark you made.

As per Woodman... Fluidmaster valves are good stuff... and they have a cheap and easily replaceable washer that replacing will very often fix the problem. If that's what you happen to have.

 
I just went through the process of upgrading the guts on our three toilets, and encountered this problem on two of the the three. What fixed it in each case was the gasket between the tank and bowl. It was letting water pass slowly, into the bowl, causing a periodic refill.