Whistling sound coming from Hot Water Cyclinder

[Conk]

Hi there,
I have a joule cyclone twin solar unvented pressurised hot water cylinder that is floor standing. Just recently, I have noticed when I turn on the hot water supply taps anywhere in the house, a whistling sound starts to come from the tank itself. It gets louder the more you open the hot water tap and there is a small vibration coming from the non return hot water supply valve which is on top on the tank itself. I do notice though, if the water isnt at a high temperature the sound is barely there at all. The temp and pressure relief valve doesnt seem to be leaking and I cant detect any air leaks elsewhere on the tank.
Any ideas to what is the fault?

 

[SimonG]

You need a g3 registered engineer to look at it for you. Nothing on those cylinders is a diy repair.

 

[Conk]

You need a g3 registered engineer to look at it for you. Nothing on those cylinders is a diy repair.

Thanks Simon, I wont be carrying out the repairs, I just wanted to gain some knowledge of what could be happening, I'm interested in plumbing and work in a similar industry.