Your boiler has detected a sudden drop in water pressure and is briefly checking if the system is stable enough to keep running.

DIY fix possible

Check the pressure gauge on the front of the boilerIf the needle is below 1, use the filling loop taps under the boiler to top it up to 1.5 barEnsure both filling loop taps are fully closed after topping upReset the boiler by turning the selector knob to 'R' for five seconds