Glowworm Flexicom 30CX Noisy?

Hi, we had a Glowworm Flexicom 30CX installed about two years ago. First it developed the leaky pipe problem which I fixed thanks to this sites advise. Now it has a second fault. It used to be relatively quiet when running however it is now becoming quite noisy.
Its hard to describe the noise but it sounds like a constant fluttery rumbly rattle as though somethings trapped inside it. The noise started about November time. We had the annual service done by a plumber who didnt comment on the noise but passed the boiler off as being fit and safe for use (suspect that maybe he chose to ignore the noise as didnt want the agro !!) Is this a common noise fault with these boilers - Is there a simple fix?
Any advise greatly appreciated.