Hi @Poppiepete,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
A constant hissing sound from your toilet after it has finished filling usually means that water is still leaking very slightly from the cistern into the bowl, causing the inlet valve to keep topping it up.
If you're confident that the seal at the bottom of your flush valve was installed correctly and no grit or grime is preventing it from sealing properly, then the issue is likely with the fill valve, which is the one on the right in your photo. Even small bits of grit or limescale can prevent it from sealing fully, leading to a constant hiss. Turn off the water supply, disassemble the inlet valve cap, and rinse or replace the rubber seal if it’s worn or hardened.
Check out step 4 in How to fix a leaking toilet for some guidance on this process.
Let me know if you have any further questions.
Jacob
