hmmmm noise from toilet

constant noise comes from inside toilet even during not used...sounds comes 24 hours...I checked any water leaking inside toilet, but it seems like no leaking...very strange....
looks like some air sounds comes from water shut-off valve which controls water IN-Stop inside of toilet sink.....this one was replaced as brand new (around $15 from home depot), so I have no idea where and why this noise continuously generated.....any idea would be appreciated...

Turn the water off to the tank.
If the water level drops, it's a bad flapper.

If the water level in the tank goes over the overflow tube, then it's either not adjusted to the right height, or the fill valve is not shutting off.