Your boiler is struggling because there is trapped air circulating inside, which is causing the internal pump to make a loud, grinding, or rattling noise.

DIY fix possible

Switch off the boiler and allow the system to cool down.Bleed all radiators in the house starting from the lowest floor to the highest to release trapped air.Check the system pressure gauge and top up the boiler loop to between 1 and 1.5 bar if it has dropped.