This might sound stupid but I've just changed the diaphragm in the toilet as it was hissing and not filling for a long time.
The toilet flushes great and only takes about 20-30 seconds to fill now, but at the end it makes a quick hiss sound.
I'm assuming this is probably the valve closing fast and that's the sound of the water flowing through but I just wanted to double check that there isn't something else wrong.
Is this normal?