Silently filling tank

The tank keeps filling but I can't see or hear the water coming in. It slowly fills over the hours. It looks like it may be coming from the bowl. After I flush, the water comes in normally and the ball rises and the water cuts off. But hours later, it will have risen to the overflow valve. I can't figure it out! It's driving me nuts! Please help!

 

Replace the fill valve. In most cases a fluidmaster 400A will fit tha toilet.

 

Thanks! Replacing the fill valve did the trick!