Check to see if the float is hitting something or rubbing on the side of the tank as it comes back up. If so, if the float arm is metal, bend it slightly to clear obstructions. If plastic take old of the column it is attached and rotate it slightly to clear. Becareful, old plastic parts can become brittle, so handle with care. If you have the float that slides up the column, check to make sure the surfaces are smooth. Finally, the filler mechanism can be replaced after shutting the water off to the toilet and draining all of the water. Replacement instructions come with the filler. It is easy.