Hi David - I just today had a Glow Worm engineer to my 38CXi bolier because I had same problem, showed him your question as your doodle bug description was spot on. My boiler is 12 months old, works fine but has made the noise since new. He could find no fault with the boiler. I had arranged that he service it as part of the visit so he stripped it down. Then when he restarted it doodle bug! He said he had never heard it before and investigated further.
Seems I have a rather long plume exhaust pipe leading above my car port outside and although within the total length specified by Glow Worm has three bends. Glow worm say nothing about shortening the pipe as an allowance for the bends but my engineer suggested that it could be the combination of length and bends that is giving the boiler a problem when it first fires on low load - he surmised the fan is not able to push against the column of cold air in the exhaust pipe so the burner shuts off then immediately restarts as soon as the gases are expelled hence the throbbing on-off doodle bug effect - there are some very clever electronics and sensors in the boiler. Once the fan is running at full speed it is obviously able to move the exhaust gases, hence I only have the problem at cold start up. I have just removed 2m of the plume pipework, it is a push fit, and ensured the anti-bird mesh is in the exit of the pipe and the problem hasn't reoccurred. If you also have a long plume exhaust pipe - try shortening it (don't forget the anti-bird mesh), if not get the engineer out.
Colin Gatward
January 2008
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