A few days ago, one of the toilets in the house developed a noisy
refill. It refills normally for a few minutes, and then it gets noisy
like there's air in the supply line.
I've observed the inside of the tank during refill, and for the first
few (normal) minutes, water comes in through the fill valve and refills
the tank normally, and there's a good flow of water from the little
refill hose into the tall white tube.
Then the high-pitched hissing noise enters the mix, and flow from the
rubber hose slows or stops. I start to see tiny bubbles coming from
the bottom of the fill valve, and water starts leaking out of the top
of the valve.
Sometimes water shutoff is noisy, and one night we had actually
hammering. Most of the time, though, shutoff is fairly normal.