Just had a buderus G115 installed and when it fires up
The first five minutes the damper erradically flutters and results in a
Somewhat thunderous sound heard through-out the house...
{reillo gun}

You better be calling the company back that installed that unit. There could be a few different causes but it is all guess work unless you are on site to actually hear and see what is going on.

That boiler is a modified 3 pass design medimum efficiency style and will soot up if not running correctly.

Hopefully they used a chimney instead of direct vent. That boiler is happier with a chimney especially with a Riello burner. Some companies don't know a lot about the Riello and that will do doubt cause problems, especially for you. That burner also does not like direct cold combustion air directly taken from the outside because it can cause ruff starts when the temperature drops.

Best to get it taken care of NOW so as to prevent more problems. If the install company does not know how to fix the unit and work on a Riello burner I would call another company that does and back bill the original contractor.

Good luck with your problem