water hammer after a regulator is added to system
You may have air chambers that drained during the meter installation and refilled with water, rather than air.

First, check that any existing air chambers are working. You may need to shut the main valve and completely drain the system for a while to get air back into the chambers. Most are inaccessible, inside walls, so are not easily replaced, but you can add another chamber elsewhere.

Also make sure that pipe brackets (hangers) are firmly mounted, since a loose pipe exacerbates the issue.