I have almost the exact same problem. I do not have a pressure regulator on the house itself, The Sprinklers are T-ed off the mainline before it even enters the house. The sound is very faint, but it is distinct. It sounds like a rythmic waterhammer. The whole time the sprinklers are running, I can hear a faint bump, bump, bump, bump,.... How can I be hearing anything at all inside the house if no water is running through the house plumbing? The valves do not seem to be chattering, and it happen...
