I have a unvented cylinder and a external Worcester boiler with good water pressure to the house naturally. I have a new system of 4 years which has plastic piping through the house. I have noticed for the first time in 4 years that (now I have turned the heating back on this autumn) there's this tapping noise coming through a false wall, where I no full well a lot of pipes run from down stairs to up or the other way round. Could the sound simply be the pipes heating up and moving, possibly moving within clip that hold them? There is no noise when the heating is off. I assume as the system is pressurised it would leak even when off. Why can I only hear it with the heating on? I have turned it on for the second time just now and its doing the same been on an hour still doing it. No sign of damp but its behind a false wall on ground level and I have tiled floor and high tiled skirting so it might be hard to tell?