if you havent already you should "desludge" your system using the approate chemical addative (then draining out the system as per the addatives instructions). Once this is done a inhibitor should bee used. After doing this (if the noise is still present) you can use a noise redusing addative.
Its not generally advised to use the noise reducer on a sludged system also for an efficient system it should bee desludged/inhibbited once ever year or 2, hense the desludge/inhibitor steps though you could go straight for the noise addative.
heres links to the approate addatives on screwfix (though most diy shops also sell them)
desludger -
http://www.screwfix.com/prods/89458/...Sludge-Remover
inhibitor -
http://www.screwfix.com/prods/46974/...nhibitor-500ml
noise reducer -
http://www.screwfix.com/prods/44452/...centrate-500ml
I beleave the ones in the links are for pooring direct into the central heatings header tank (not the cold water tank, bee sure you get the right one
). If your system dosnt have a tank you will need the "injectable" versions (which can bee added via the filling loop or direct into a radiators bleed valve) but these tend to bee more expensive.
You should also bee sure to check the addatives are suitable for your boiler (though they are in 99.9% of cases some types of boiler needs different addatives)