Approximately every hour to 2 hours, there is a noise coming from the cupboard where the hot water cylinder is. It sounds like a loud release of air, I can't locate exactly where it is coming from as I am never there when it happens. It is not continuous, just a loud noise for about a second, a big "whumph". About 1 or 2 months ago we had the wetback re-connected to a new logburner.
I have turned it off, the pipes were hot and hot water is coming out of the tap.