Baxi Bermuda 552 bubbling and clattering?

Hello, Just joined. I value any ones advice here please.

I have a Baxi back boiler in the living room inside the fireplace. The primary circuit (from the heat exchangers to the cylinder) seems to bubble like mad and occasionally clatters waking me up at night and irritating me during the day. This I want to cure because there must be air in the system and I do not know how to 'bleed' it.

The HW must be on to work the CH. Both produce piping hot water happily but the problem noise (and I guess poorer energy efficiency) does not go away.

I can not seem to find a bleed valve anywhere on the primary circuit to or on the pipes leading to or from the cylinder. The header tank was cleaned and the CH system was thoroughly drained and flushed and topped up with Fernox during the summer. The radiators were (eventually) bled fully by late October, I have not needed to do these since and the rads appear fine now. Recently I have noticed that the header tank now has a black scum on the surface of the water.

Much appreciated.