Why does Baxi solo 250 pf pulse and cough?

I have a gas powered Baxi Solo 250 PF boiler which is around about 10 years old...

It has recently started to make a clicking noise when the boiler is trying to ignite (when heat is called for).... It doesn't happen every time, sometimes it fires up straight away with no noises.....

Many thanks for any suggestions you can make.