Noisy boiler?

I have a Keston 175 boiler, about 5 years old, which has started, over the last few months, making a lot of noise. I can only describe it as a long drawn-out \\\"moan\\\"!! A bit like a light aircraft in the distance (but louder). The noise starts without any warning and lasts about 15-20 seconds with the note sometimes rising and sometimes falling. It is loud enough to wake us up. The system is non-vented. Can anyone help please??

dont know that boiler but that sounds like classic heatexchanger scale. can u add some x200 descaler.

Thanks for the suggestion. I'll get some tomorrow & "administer" it.

Cheers