For many months we have had a whistling sound when running water but it was not audible when the freshwater pump started. The cause was a tiny leak from a hole in the pressure vessel located outboard of the hot water cylinder. This is made of steel and connected to the system through a brass elbow and copper pipe. The vessel was badly corroded and a temporary repair made by grinding off the paint around the hole and applying several coats of Araldite. An identical pressure vessel was bought for 11 pounds sterling in Malta from a shop supplying pumps etc for irrigation and we fitted it with a plastic elbow between it and the brass/copper fittings. The problem was,I assume,caused by electrolitic reaction. There is a 2 litre stainless vessel on top of the freshwater pump and the one behind the hot water calorifier is 5 litres. I suppose its function is to cope with water expanding whilst it is being heated although there was only one vessel in the water system on my last boat,an Oyster.It was one of the few things that never gave trouble in nine years of liveaboard cruising. Does anyone know whether this second tank is actually needed and does it have to be so big as stainless tanks are only available here in 2 or 20 litre capacities. Anne & John Hollamby