I know this is a poor boiler but I have one and I need help! lol. I recently had 2 new rads fitted due to an extension. Although my heating and water is working fine and the boiler is currently operating, for some reason every 4 to 5 mins, the boiler is making a strange noise. It's like a large hissing noise as though it's letting air out or revving up somewhat. I haven't a clue why it's doing it and don't really know where to start. I've looked in the booklet and tried the troubleshooting but am not getting anywhere. I just don't want my boiler to die on me the week before Xmas! Anyone get any suggestions? Many thanks!