If you mean non-stop hissing, you fix the leak, of course.
I usually rely on a plumber.
You could reduce that hissing sound in the toilet a lot if you would just drink less diet soda, Cowboy.
First of all, I would try to find out the source of the hissing sound - with precautions, of course.
Sometimes, surprises can be nasty or cute (I'd prefer the cute option):
Although this MIGHT be hard to take,
In Cowboy's case, it was a snake.
I had indeed an awkward suspicion,
a smell of perdition - or pernicion;
referring to past and new tragedies,
hidden behind weird gags and comedies ...
Call a plumber