My Baxi sounds like a loud electric kettle - Whats wrong?

My 6 year old Baxi back boiler is making a noise that sounds like a very loud electric kettle when it boils. It also sounds like there is soot or something on the burners. Any ideas as to what is making this noise. It has made the noise before in the past but after a few hours has righted itself and been OK for months. Now it does it all the time. I have just joined a service plan, but they cannot do anything until they have inspected the boiler on January 19th. Any ideas as to what is causing this problem would be welcome.