This is most likely going to require an Gas Safe engineer to visit and inspect the heat exchanger and flush your system.
However before you make the call, its worth checking the outlet valve is getting hot which should be from the top of the boiler its the pipe on the left (nearest to the flu - the one on the right is the return). If thats getting hot the pump, heat exchange and burner are all functioning. however the noise is likely to be caused by a build up of hard water and iron oxide within your system, this can often be fixed by flushing the system a few times with a cleaning agent, which removes iron oxide and limescale.
If you want to flush the system yourself you may find the video here useful, please let me know if you have any questions from watching it.
The video explains two methods which relate to two different setups a vented or a sealed system. A vented system is often found in older installs, and has a filler \ header tank in the loft, the other method uses a expansion tank and works based on pressurising the system, and you should find a pressure gauge and filling loop near the boiler if this was the case.
Please also let me know if any error codes have been displayed on the boiler over the past few weeks, as this may also point to something else you can check.
Should you call out an engineer they may offer you a water filter to stop re-occurrence, however depending on the age of your boiler I wouldnt have it done, unless your in a hard water area and the boiler is less than 5 years old.
Please let me know how the results of checking the tank \ pressure and outlet pipes, or if you need anything clarified.
PS: You may find it useful to read the attached installation manual for your boiler, which can be found here.