I have studied many posts regarding noise but non seem to cover my problem. About once or twice every other day I get a deafening foghorn noise which seems loudest in the airing cupboard. It rises to a crescendo then after between half a minute and one minute is fades away. I once tried switching off the boiler and the controls but the noise continued as before. I'm wondering if there is a pocket of steam somewhere or a ballcock problem since the noise is present with the heating/HW off.