Some inlet valves have a tube that is a screw fit into the water outlet into the cistern and sit just above the base of the cistern so it's filling underwater which eliminates noise have words with local plumbing shop.
Sue's idea could help but will slow down the filling, so the choice is yours if you haven't already cured it.
Bob, October 2008
We had this at our old house, the water pressure was very high living at the bottom of the hill. We reduced the flow of water to the house by turning down the water stop tap and turned down the flow to the cistern as there was a valve to isolate the toilet. (not all toilets have these)
