Sounds like it's just the valve starting to close off the supply as the cistern is filling. The water is flowing through an aperture that's getting smaller all the time and is producing the noise. You might find the same effect with a cold-water tap in the bathroom or kitchen, with a difference in the sound starting from from full flow to when you gradually turn the tap off.
Take the lid off the cistern. You should see a float with an arm attached to it. It's the other end of the arm that is operating the valve controlling the water flow. As the cistern fills, the rising float will reduce the flow until the flow is eventually cut off.