glow worm swiftflow

Treat me gently, its my first time...
I have a glowworm swiftflow combi boiler,hot water ok but boiler makes chattering noise after a while, also the other day it made a rumbling noise and looking through the peep hole at the pilot instead of the normal icy blue flame it was a very sulphurous yellow colour, all didn't seem well so I turned the ch off although hw supply is fine.
Am attempting to get some out to take a look but in the meantime any thoughts!!

Thoughts are that you have a combustion problem.

This being the case, unless you have a birds' nest in the flue, then it would be immensely unwise for you to act on any advice other than the recommendation that you call a CORGI registered engineer to diagnose the problem.