my mistake, I couldn't isolate the high pitched whine in the cupboard but now I am sure it is coming from the pump/motor area at the side of the cylinder, perhaps the motor is on the way out
its definitely coming from the top where the immersion is and it is red hot to touch and so is the pipe into the cylinder and that is with the power and the central heating all switched off
I have an immersion heater screwed in to the top of the cylinder. Even with the power to the immersion switched off there is a high pitched alarm coming from it. This tends to happen when I switch the central heating or the shower on
