I've noticed lately that when I put the central heating on, there is a tapping sound coming from where the gas meter is. My gas and electric meters are in a cupboard in my living room (small terraced house). After the heating has been on for an hour or so, there is a tapping sound coming from that cupboard. It's very intermittant - sometimes constant tapping, other times nothing for a few minutes then starts. I thought it might have been water dripping from a pipe (as it sounds like that), but can't find any water near the pipes. And also it doesn't happen all the time so can't be a leaking pipe. What could it be?