Dear Newsgroup,
I think a shifty plummer pulled a fast one on my 82 yr old mum.

As soon as he left the water tank behind the toilet started acting very wonky.
The water goes up and down...up and down.....up and down.
Each cycle jolts the pipes throughout the house making sleep impossible. its on a timer or something...functions OK for a day or 2.
Then the up and down starts in again. The black flap on the bottom of the tank is shut tight so its kinda spooky watching this.

Any illumination would be tremendous, thanks in advance!!