Your boiler is struggling to circulate water because trapped air is causing the pump to run loudly and inefficiently.

DIY fix possible

Switch off the boiler and allow it to cool completely.Locate the manual air bleed valve on the boiler's air separator or nearby pipework.Using a radiator key or screwdriver, gently open the valve until water flows steadily without air hissing, then close it securely.Check your system pressure gauge and top up to 1.5 bar if necessary.