Your boiler is reminding you that the water pressure inside the system is getting low and needs to be topped up to keep it running efficiently.

DIY fix possible

Locate the external filling loop, which is a silver braided hose connecting two pipes underneath your boiler.Open the valves at both ends of the hose to allow mains water into the system while watching the pressure gauge.Close both valves firmly once the needle on the pressure gauge reaches the green zone, usually between 1.0 and 1.5 bar.