When our heating comes on in the morning, the pipes to the rad in our bedroom make a loud knocking or slapping noise. It goes for about 20 seconds before slowing down and fading out. We only noticed it recently as the heating has been on for longer due to the colder weather. The system is only 6 months old so we can still call the plumber that installed it but I thought I'd see if anyone had any ideas first. Thanks in advance 
