I have recently drained the central heating system several times as I have been replacing some old radiator valves. The boiler then began to make a crackling/creaking noise when in operation which I think was "kettling" due to a build up of sludge in the system I may have disturbed. I got a British Gas engineer to have a look at it but of course when he came the boiler behaved itself during his brief visit and he could find no fault and blamed it on it's age etc. He serviced it and gave it a clean bill of health. I then emptied out the system myself a couple of times and added some Fernox Restorer which stopped the noises.I left this in for a week before flushing it out a couple of times and adding some Fernox inhibitor. The day after I added the inhibitor a different wailing/whining noise began to come from the boiler after it has been on 15 minutes or so. I've vented all the rads and they all get hot. I had the boiler fan assembly replaced 11 months ago after it's motor went and I think a similar but less audible noise occured not long before it failed. It couldn't be that again surely?
