Boiler "moaning" noise on heating water

[UserError]

Hello,
We have relatively recently moved in, and from the start, the boiler randomly makes a long moaning sound for maybe 30-45 seconds. I have uploaded a recording to ttp://www.spidery.talktalk.net/misc/boiler1.mp3

It only does it when the controller calls for HW. I have not yet been able to provoke it with a CH demand.

The system was under a BG contract & serviced regularly by the previous owner. For some reason, we didn't continue with the BG contract. We then had a local engineer round who was somewhat vague & advised us to "live with it". Haven't had him back either...

Getting a bit fed up of being woken by the noise when the HW comes on at 5.30.

Boiler is a Potterton Promax 15 Sl on a Y plan system, which we've been told (by a different engineer who came for an unrelated job) isn't all that old.

I have searched this forum & the web, but most boiler noise is banging/kettling but this is different.

If anyone could give me any pointers or advice I would appreciate it. I am not against getting professional help, but that which we have had so far (of those who could be bothered to get back to us) has not been helpful

 

[Dotty]

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