New gas boilers are 100% more efficient than the ancient boilers and heat up much faster. The quick boiling can cast water up into the mains creating banging waves of water. Reducing the firing rate to make it heat more slowly is not an option, so something must be done to separate the moisture from the steam being driven from the boiler.
Check the Flash pictures in the Steam Tips section of
http://www.hydronic.net
to see if your piping looks like the ‘better’ piping example. If the piping looks more like the ‘bad’ example, you may have found a cause that would need correction.