FLL, is 100% correct the flapper needs replacement. Exept the seat will be plastic on any toilet valve that has been manufactured since the second world war at least, but it still needs a wipe down with your finger. The rubber in the flapper breaks down and cannot hold air within the pocket (the hollow plug under the flat lid) and it loses its flotation and therefore slaps down. Honestly, due to experience in a past life maintaining apartments, the cheap Yellow Hornet brand flap that is at Aubuchons is the best fit for the fix (as long as it is an American Standard or old Kohler style flap), I have tried all the different models that were available approx. 5 years ago, we had 240 apartments under our ownership and I cannot even remember how many tiolets I had hugged in the time spent there, I shutter to think about it still...
