I've got a constant hissing sound coming from the left side of the tank. Similar to air being pushed/sucked. At one point, too much water was in the tank and a small trickle was noticed too. But that has since stopped after I took the cover off and flushed a few times.

Taking the cover off, this sound seems to be coming from small "vents" between where the filler float sits up top and the cap of the filler valve. I can muffle the sound by putting my fingers over the holes. I also tried shutting the water off to the tank, once doing this the hiss stops.

Please advise.