Sometimes the surface the flapper rests on can have a flaw that allows water to seep past. I had this happening on one of my toliets. So I got the Korky brand flapper with the "flapper surface repair kit", its essentially a plastic circle that is ringed on top for the flapper to lay on, and you attach it to the old surface with a silicon adhesive. I installed mine a while ago, and now the toliet doesn't leak a drop! No random water-fill sounds anymore.

My other toliet I just installed, replacing the 60 year old unit that was original to the house and sucked at flushing effectively. (an old Briggs) New one is a glacier bay with the dual flush system with buttons on top of the tank. The mechanism in the tank is an engineering marvel. And it has a teflon type gasket that sets on the opening, and is guaranteed to never leak. So far so good.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)