My central heating system uses speedfit pipes. When the heating is on, there is an intermittent creaking/ticking noise from the pipes. I have narrowed it down to a couple of connections - a 28mm elbow and a straight connector. What exactly would make these creak? Should I replace the whole connectors or just the inserts or the actual pipe?? Or can I just lubricate the joints somehow? If so, what with, and on which parts exactly?