noisy worcester bosch 24i boiler?

when the heating comes on the boiler and pipes make a loud gurgling and popping sound as if there is air in the boiler and system and the pressure gauge begins to twitch. This lasts for about 15-20 minutes after which time it settles down to just a soft dribbling or 'kettling' sound interspersed with a soft low knocking type of noise the system has just been cleaned out and air vents have been fitted to it.The hot water system is fine but lately i have noticed scale coming out of taps

sim
January 2007