Information about my house: The house is up on a hill, probably 25 feet vertically from the street. Water comes into the house in the basement and tees off for the sprinklers BEFORE going to the pressure reducing valve for the home (irrigation system is getting full city water pressure). Irrigation line has it's own valve before going out of the house and out to the valve boxes for the sprinklers.

Whenever I turn on the sprinklers, the first valve opening creates a loud knocking sound coming from where the pipe tees off from the main line. It only happens when the first station comes on, after that it's perfectly quiet as it changes from station to station. It doesn't matter which station I start with and the knocking lasts 1-2 seconds and stops.

I had a water hammer arrestor installed right where the irrigation line comes out of the house, but before it meets PVC, but that doesn't appear to have helped much. I think my next idea is to add a pressure regulator on that same line, but I'm curious to hear if anyone has any other ideas I should try first. I'm also a little bit concerned that reducing the water pressure will affect the sprinkler heads and how they perform.