need help with worcester danesmoor wm 12/19 lock out etc?

Hi Need some help!

Runs fine heating hot water.But When heating rads it goes to lock out quite often, which is a pain. It also makes some strange cracking, crackling or tapping noises when the heating has been on for 5 to 6 min, which drives me round the twist, sound from top of boiler? but no noise when heating hot water only. Boiler man has put descailer in 2 weeks ago but no difference yet. Any help?