What is that hiss coming from my toilet long after it has flushed?
you probably have a seal in your flush valve assembly that is
shot and not closing off the water when the toilet fills back up.
Or you have something like the chain if an older assembly sitting
under the flapper keeping it from closing all the way. If you're a
DYI person it would be best just to replace the assembly, it's
pretty easy to do ... plus if it is the assembly, replacing it will
save you on your water bill too ...