Why don't I ask the same plumber? I asked the same plumber, who then wanted to charge me £600 to flush the system as "the fernox had released the sludge in the system, which is causing the noise". I am thinking he may not be the most reputable source ...
This boiler has always been noisy and on the recommendation of a plumber I dosed the system with fernox. What a result - silent for a few weeks but now it sounds like a steam train! Chuff chuff chuff. Any ideas?