In 2005 I added an addition to my house which included a laundry room and master bath. I don't know if the plumber installed any water chambers. I have knocking when the bath, toilet, sink or shower is turned on. The knocking seems to be centralized under the laundry room. We have secured every pipe that even appeared to be loose. Is there a way to install water hammer arresters without having to open up the walls? If so, where and how many?

Sioux Chief makes a hammer arrestor that will thread onto the hot and cold outlets at your clothes washer connection.

The one you want is the top left one in the photo.