To determine if it the piping, have the technician run the circulator without the burner running. If the bang is from the pipe arrangement the bang should occur.
Sometimes castings can have small pockets where steam is produced and then the water bangs in the casting when the steam bubble collapses in cooler water as the bubble expands out of the space. Your technician may change or stop the bang by raising the boiler water pressure to see if the bubble can be forced to stay in the cavity.
Changing the speed of the flow of water within the boiler may eliminate the bang.
One of these culprits is the cause, so you need an experienced boiler plumber to check this for you. It is not a do-it-yourself problem.