Its a indication that the piece of rubber diaphragm has torn up. Its time to change it. You will need to open it up.
You could use a plastic sheet ( transparency sheet will be fine) and cut it out to the required shape.
Make sure its of the same size of the original. Alternatively you could buy the diaphragm for a few dollars.
You will have to take a look into the cistern and figure out how to dismantle and get to the diaphragm.
