Sounds like your plumber did not put air dampeners into your pipes,
which are risers teed of your line so that air can absorb the
vibrations and knocks.
Besides being annoying, it's probably not a huge deal. Except that if
he missed putting in these risers, what else did he miss? If you can
access the back of the plumbing through a closet or something
convenient, you can look at it and fix it.