By a strange chance I have a plumber in my house at the moment replacing my water tank, this is what he said. The noise might be vibration in the washer. Shut the water off and remove the three screws securing the top of the ballcock. Pull the float ball ands stem and washer assembly up and out. Pry the washer out and turn it over pressing it in in firmly. Place your hand over the open ballcock and reach down and turn the water back on to flush out any crap that might be in there. put it back together and give it a flush
If you have a plastic mechanism you might just need to fiddle about with the pins instead.
Hope it works, if all else fails get a plumber in!
