Sludge in the Central Heating Boiler

Hi,
I have Glow warm Central Heating Boiler and it makes a big noise some times. I called in the British Gas Central Heating man and he says that sludge settles in the heat exchanger area and when disturbs, it makes big noise.
I do not believe in his version as the noise is of gas burning with delayed action. Gas gets accumulated and then burns like the one in Gas Ovens.
Any idea of getting rid of the sludge?
Rajinder
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Flush the boiler?

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Like a delayed ignition? Thats bad news...

If you do not believe him, why are you using him?

Normal pre season service???

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