Your boiler is reminding you that it is time for its regular service because the water pressure has dropped slightly outside of the ideal range.

DIY fix possible

Locate the external filling loop, which is usually a flexible silver braided hose under the boiler.Slowly open the valves on the filling loop to allow water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar), then close the valves tightly.