I have an intermittent moaning sound coming from my Worcester Bosch Greenstar 25i gas boiler. When it occurs, all I have to do is open the hot water tap in the kitchen, wait a few seconds, then turn it off. While I turn the tap off, there is this moaning sound. It has only happened occasionally since I had the boiler installed less than a year ago but I would like to know, if possible, what it could be due to. Thanks very much.