Potterton boiler clicking noise

My Potterton netaheat has been a good boiler (replaced pcb twice in 20 yrs). Now when fired up and runnng it occassionally starts manic clicking as if trying to light but is already running fine. The sound seems to come from an area behind the pcb. It clacks away intermittently and then stops. It is not affecting the operation of the boiler at the moment but this definately aint right. Any ideas.

has it been serviced recently? Sounds as if flame rectification is not getting the signal to pcb that pilot is alight therefore starting spark/ignition sequence. Could just need a good clean