Glow Worm Spacesaver bangs & crashes when starting?

I have a wall hung 65000 BTU balanced flue boiler in our kitchen, it came with the house 25 years ago (when new). The system is pumped (pump is in an upstairs airing cupboard & quite a way from the boiler) and directed through a 3 function valve to either or both the radiators &/or an indirect hot water cylinder as required.
When either the heating or hot water starts from cold , the boiler bangs & crashes for around 4 or 5 minutes and then continues fairly quietly for the rest of it's operating time. The rads all get hot and I flushed the system around 3 years ago followed by a refill topped up with Fernox (or similar). There is a dead-leg close to the pump which collects a little air now and again but the rads don't seem to get any air trapped in them.
All in all the boiler seems to work fairly well but the noise is quite bad, and can be heard all over the house. - Regards