My parents Potterton Suprima... They've having some building work done which required drain down and rerouting of the boiler flow and return pipework. (Plumber did open boiler casing looking for a drain c*ck for the boiler, but it seems there isn't one and the plumber who installed it
10+ years ago didn't fit one.)Anyway, boiler is now making a new noise. When I first heard it, I assumed it was the builders running a cement mixer outside. When they'd gone, I realised it was coming from the boiler. Initially thought it must be fan out of balance and drumming on the case. Took the casing off and started up the boiler (with the gas supply off so it doesn't light), and the fan is fine. Checked the little cooling blades on the other end of the shaft are not making contact with the case, which they weren't. Put it all back together, and the noise is gone when the boiler fires up, but only for a minute, and as it runs the noise slowly comes back. Actually, I realise that as this cycles on and off (it's not a modulating burner), the noise builds up slowly over that period towards being quite loud by the time the burner cuts off. I already checked the pump, and there's no vibration there or in the pipework near to it (it's some way from the boiler, being in a different room).
My best guess at the moment is that it's a resonance which is setting up in the flue. It does sound a bit like a giant church organ pipe. I think it's far too uniform to be any sort of kettling. There's enough power behind it that you can feel the vibration on the upstairs floor above it.
However, I can't imagine what would have caused this to start happening. Nothing has changed with the boiler or flue positioning. It's possible the boiler might have sucked in brick dust (giant angle grinders have been in use rather near the flue terminal), but that was not obviously visible under the burners, where there were just a couple of dead wasps and spiders (I had serviced it nearly a year ago). I have not looked down the flue pipe to see if there's any obstruction, which I'll try and do next time I'm there. Apart from the noise, the boiler is actually working fine.