I have a Kohler toilet, installed in 2000. Recently it has been making a flushing ( hissing) sound from the tank. The plumber has been here 4 times. He replaced the flapper, vacuumed the tank, did the dye test, changed the float, and finally replaced every component in the tank. It is still making the noise. Any suggestions?

vacuumed the tank,

Could you tell what that means.