Radiators (banging in pipes)?

We installed a new baxi boiler several years ago (to upgrade from a very old baxi) and ever since, we have had to a more or lesser degree banging in the pipes at or around the boiler area. We put in a new hot water tank to try and cure it but it is the same and have bled the system several times and the water always runs clean so there is no contamination in the system.

Could this be a problem with the boiler staying ignited for too long? even though the pump is running fine it seems the banging is directly at the boiler. Is there a special way to bleed the system to completely eliminate every bit of air...?