Maxi combi 80e 'fan' runs intermittently?

Good morning,
My 10 year old approx Baxi Combi 80e has a characteristic where it periodically makes a fan type whirring noise. The noise happens perhaps every couple of hours or so, and lasts about 15 or 20 seconds.

There is no demand for heat either from the CH or DHW when it does this.

I say fan on the assumption it is that. Its the same noise the boiler makes initially during the startup sequence. (Normally this is followed by the whumpf as the gas ignites)

Pretty sure it's not a preheating model.

I have an engineer booked to service it and I'd like it fixed if it's dangerous, but I'm happy to live with it if it's not a problem and / or very expensive to fix.

But I imagine it's difficult to diagnose if it doesn't happen when he's there!

Also, I don't want o be baffled by BS and end up getting taken for an expensive ride. (I've not used this company before)

many thanks for you help

Jez
October 2016