Completely different problem than a recent post, and a problem that I've had
for quite a while now...

Baxi Solo 2 40/PF Boiler.

When the boiler is supposed to fire up, the boiler can go through a phase of
firing up the fan, then cutting it off a second later. This results in a
quiet "whoophm, whoopm, whoopm" noise (around 0.5Hz) for the few minutes
before "something" takes hold, the fan spins normally and the boiler fires
up properly (with a healthy "click" of some relay somewhere actuating)

During all this, the proverbial "tap" on the side does cure this (boiler
fires immediately).

So.... what's this likely to involve?, is there some protection circuitry
around the fan that might be goosed?

As always, any help appreciated.

Mike