Help, please, with intermittent loud droning Vaillant boiler which they can't fix - link to video

[Roddy Young]

Can anyone identify the source of an intermittent loud droning noise from our Vaillant EcoTec Plus 612 boiler?

Vaillant’s engineer came out but it didn’t happen that day! He did check the boiler through and found no faults.

Before I ask a local repair man to have a look, I wonder if any of you might recognise the symptoms.

It is loud enough to be heard through the house and can be felt on the boiler casing.

In this video, the underfloor manifold was flowing, the tank coil was not. It made no difference when I opened the tank’s valve.

The noise stops as soon as the “flame” button is pressed at 38 seconds. Does that suggest it's a flame fault?

This only happens a couple of times per month. There may be some connection with wind – it was windy outside today when I filmed it.
I was plagued with this some months ago but it seemed to have settled down. Now it's back!

The boiler heats an underfloor heating manifold and the hot water cylinder, each through a 2-port valve.

Any thoughts would be much appreciated.

 

[Johntheo5]

If you run to the HW cylinder only is the noise the same?. Have you UFH only as heating, no rads?.

 

[Roddy Young]

There is only UFH.

The noise is the same when only the tank is being heated without the UFH being on.

Is it a coincidence that it has done this a few times today, the first day with blustery wind hitting the gable where the balanced flue exits?

 

[ianmcd]

There is only UFH.

The noise is the same when only the tank is being heated without the UFH being on.

Is it a coincidence that it has done this a few times today, the first day with blustery wind hitting the gable where the balanced flue exits?

You dont have a balanced flue, you have a concentric fanned flue, definately not the same thing

 

[Roddy Young]

That's interesting, Ian, does that mean it can be upset by gusts of wind hitting it head on?