noisy boiler?

hi
my boiler is making a knocking sound...i have drained the system and bled all of my radiators they all heat up fine and have no air locks...i want to administer Fernox Boiler Silencer Superconcentrate but i do not know where you put it into the boiler,i know i can put into the expansion tank but i want to put it directly into the boiler...my boiler is an ideal classic ff240 , if you can help that would be great
thankyou