Heatline C28 Condensing combination boiler.
Regards commissioning, a Corgi registered friend of mine installed the gas; of which I'm guessing you are referring.
Back to the problem.
When it was first commissioned, the heating system was all filled, pressurised and checked, boiler bled, condensate trap filled to required limit and so on. Fired her up, all worked and is still working a treat.
Hot water was tested a while after and worked also, then from nowhere when the hot tap was used a howling sound could be heard. Worse if a thermostatic mixer shower was used.
Checked the boiler whilst this was happening and without doubt it was coming from it as no pipes were knocking or moving due to being well clipped.
Along with the haunting howling sound the domestic hot water pipe was pulsing but the water appeared visually steady.
The sound could be stopped by turning down the heat which is what baffles me.
It did this for the first and second day then stopped completely by itself.
Week or so later, it has done it again..
Any help without appearing condescending would be appreciated.