Joker Valve
I would guess it's some buildup on your joker valve allowing a little bit of seepage back into the toilet bowl.The joker valve is a rubber part that acts as sort of a check valve, that is supposed to allow one way movement of fluid (waste) out of the bowl. If a little bit of buildup occurs, it can perform most of its function, but will allow some seepage back.Reasonably easily replaced, maybe a little messy, but pumping your tank and then running a bunch of fresh water through the bowl into the tank should leave you with only a clean water mess when you disassemble the valve.Rebuild kits can be purchased for most all head models. They always include the joker valve.Good Luck!Tim BroganApril IV C350 #68Seattle