Why is the overheat light on all the time when the shower is off?

Every time when the electric Redring Plus Extra shower is turned off after about 3 s a buzzing noise lasting about 5-10 s occurs, as if the water was boiling inside, and then the "Overheat" indicator turns on and stays on even though the shower is turned off. Also from time to time the water in the shower apparently boils itself inside despite the shower being turned off, and the "Overheat" indicator
stays on all the time.