I had/have a very noisy HW Cylinder. There is a constant sound of running/sloshing water. I drained the cylinder (by turning off the water supply to the cylinder) by turning on the HW taps in the house. Once empty I refilled the cylinder and hey presto the noise stopped ...... for only 24 hrs. I think there must have been air in the cylinder, which I got rid of, but now it has returned, how do I permanently solve this problem??? (There is no bleed valve on the cylinder)