noisy boiler - possible sludge problem

I have a potterton performa 24 boiler and it has been noisy recently, called out heating engineer as I have a central heating protection plan - they are having to put a new exchanger in and say the damage was possibly done by sludge (they will be giving me a quote for flushing out the sludge when they come to change the part), they say the boiler is overheating hence the noise.

I'm slightly puzzled about this as the heating system was cleaned out last year and an inhibitor was added - I spoke to the plumber who did this job, and he doesn't see how if could be sludge in the system, and the part is probably damaged due to wear and tear (usage of the hot water).

Anybody out there got any thoughts on this as I'm not sure what to believe and don't want to end up spending hundreds of pounds on something that isn't needed, thanks.

Ask to see thw water when they change the h/ex, if this is very dirty you might need a flush of some kind. You could ask about a filter on the system, which are readily available.

could you be more specific on the noise it has been making ?

the noise is a sort of groaning noise - like a trombone - the engineer said it was making this noise because it was overheating. Hope this is helpful as I didn't wait to listen for too long as it sounded unhealthy although the engineer said it was safe I ended up switching it off, still waiting for him to come back and put in new part.