background - sprinkler system was put in about 1996. Water hammer started about 3 years ago. Sounds like a foghorn. Originally when it started it went off odd hours but only at night. You don't hear it in the house - only the back sprinkler valves. * Replaced toilet valves and flappers * Turned off washer, dryer, dishwasher etc to see if they played a apart * Rebuilt PRV It is set to 60 PSI at rest and drops to about 50 psi when significant water is flowing like washer on warm. City pressure before the PRV is about 100 PSI. We now have water meters between the main valve and PRV but that was put in about 10 years ago so it predates foghorn. * Installed a pressure gauge so I can see it easily anytime I am in the garage. * Installed smaller water hammer arrester near kitchen sink because those pipes would hammer one time when faucet turned off before that arrester and expansion tank were installed. That hammer is gone. * Installed expansion tank (ST-5) sized for 50 gallon water heater on cold supply side near water heater. It is properly pressurized for PRV to 55 PSI which is what system falls to when light water is used like a single faucet. Highest volume sprinkler line or clothes washer on warm drops water pressure to 50 psi. The pressure on the gauge never climbs above 60 PSI that I can catch it and I have looked at that gauge a thousand times now. The PRV appears consistent. * As I recall PRV is installed before the water take off because sprinkler valves can't handle 100 PSI, and drip can't drop 100 PSI on drip regulators. *Installed all new 12 sprinkler valves. The foghorn has not gone off on its own since then but it does so when a sprinker is turning on or off. Usually #2. * Considered that a Ham Operator down the street is overloading the Rainbird Controller and causing it to chatter when his antenna is pointed this way. Odd hour only at night foghorns might be his signal causing the contoller to pulse 24V AC to a valve randomly. Starting to reach now. *Considering making a water hammer arrester per a plumber's suggestion at Lowes which is how he has stopped this on other systems. Online reading suggests a vertical pipe without a bladder is an old way that is no longer acceptable. * I have rebuilt several faucets, made sure no hoses leak when this happens. I have never been able to catch it doing it and when it wakes me up at say 2am making a foghorn by the time I get out there it has stopped. That is why I replaced all the valves. Anyone want to lend some bullets?