Your boiler is currently going through a standard starting sequence to clear the air before the burner lights up.

fan pre-run

What causes this fault?

This is a normal part of the boiler's operation and occurs every time you request hot water or heating. The boiler’s fan starts spinning first to clear out any leftover gases from the combustion chamber and to prove it is safe to ignite the burner. It is a standard safety check rather than a mechanical failure.