Hi. I just moved into a new place. Sometimes after flushing, the toilet cistern begins to vibrate loudly and rapidly (about 200 times a second, I guess.) Inside the cistern there is a sort of plastic bubble, and when the water in the cistern rises to a certain level, the bubble gets dragged up and stops more water from flowing in. When the vibration occurs, this bubble also vibrates, and if you nudge the bubble a couple of times, the vibration stops.
Has anyone seen something like this before? I would be grateful for any clues as to how to fix it?