It sounds like the toilet overflow, the water inlet is controlled by a float valve, I’ve added a picture of a typical one. A ball float is screwed on the the left hand side of the black arm, the right hand side is hinged at the blue valve.
When the toilet is flushed the ball float will drop in the cistern allowing the white plunger in the blue valve to come out and will open the valve. As the cistern fills the ball float rises and pushes the plunger in shutting the water off, this cut off level can be adjusted with the screw that pushes the plunger in, if you undo the lock nut and screw it in to suit the retighten the lock nut when done.
It’s best to check if the valve is working first, take the top off your cistern and gently lift the ball float up to confirm the valve is actually working and is turning the water off.
Daughters motability car Hyundai Tucson Ultimate self charging hybrid Serenity White Moss Grey interior.
My car SEAT Ibiza SE Tec 1.2 petrol TSI 2017.