Has a gas registered fitter round to service the boiler but he had to leave and go to an urgent job. Took this photo whilst the cover was off, as I am hearing a bit of gurgling from the trap every now and then when the boiler is in operation. It isn't showing any fault codes but every half hour or so when it's on, you hear the trap draining and gurgling a bit. Is that normal? Its a 2 year old boiler.