I had an irrigation system installed by a local professional in June of this year. If I turn on any plumbing in the house while the sprinklers are running my pipes in the house will start chattering for about 20-30 seconds and then settle down. The hammer was initially pretty violent but I've tweaked back the inlet valve to the backlow preventer a bit to keep the house from jumping out of its skin. I still can't get rid of it entirely although cutting back on the backflow preventer has helped.

Pressure off the street varies from 80-100 lbs. Pressure to the house is a constant 60 lbs after the PRV. The irrigation is tapped into the line after the meter, but PRIOR to the PRV.... FYI - I've also got a thermal expansion tank in the house on the cold water line (after the PRV). From what I've been seeing on the Web, the no-brainer fix would be to install a 2nd Pressure Reducing Valve in the irrigation line immediately after the backflow preventer. Whatcha' think? My plumber was also considering going with a 2nd expansion tank on the city side of the existing PRV. Your thoughts?