Another noisy boiler problem!

I have a gravity fed hot water/ heating system in a standard three bedroom two story house. The header tank is in the loft, an ideal standard wall hung boiler is fitted downstairs. A grundfos pump, three way valve, hot water storage cylinder all stored in the airing cupboard upstairs. The system has been installed for about eight years and has been trouble free. Now for no reason the last couple of months have been a nightmare. When the heating and / or hot water comes on the boiler / pipework makes a loud banging noise about every minute all the time the boiler is on. I have checked the header tank which is full, have bled all the radiators for air, bled the bleed valve on the pump run next to the hot water cylinder, turned the pump speed up from 2 to 3, down from 2 to 1, tuned the boiler up and down from 1 to 6, all of which made no difference. I am all out of ideas, any help or advice would be greatly appreciated.

I hope you've read the other threads on this subject.

Try draining some water from the system and see the header tank level goes down and then tops up. If not there's a blockage in the feed pipe.

Otherwise it's scaling in the boiler, some boiler silencer chemical should help, but it may be more serious and need the system powerflushing.

Thanks very much for your advice, I drained some water from the system and the water level did drop in the tank, so no blocked pipes at least! I then drained down the whole system and refilled it and put in some fernox descaler and sludge remover. Will leave it running in the system for about 1 to 2 weeks, and then I will drain and refill and put some fernox inhibitor in, lets hope it works! Just for your information the system is eight years old yet the water that came out was quite clean, being only light brown in color? I expected it to be horrible? Fingers crossed that it works.