You isolate - test - eleminate. Turn the system off for a day or 2 and listen for the leak at the same place you are hearing it now. If you hear the noise look toward leaking main or other source.
If you don't have a MV and the sound of water is not present when the system is off and the meter is still or pressure stays up (well doesn't cycle) - eleminate a leaking main line -
With a MV you can listen to the valve with a sounding stick or piece of pipe. No noise - eleminate MV - Listen to all of the valves, no noise or meter movement/cycling - eleminate valves -
Plug or cap all but 1 head on the suspect zone. Attach a pressure gauge at the remaining head POC. Activate the valve watch for pressure drop while listening to the valve. If the valve is silent and the pressure doesn't bounce around - eleminate the lateral -
There are many places where a leak could be but if its in the irrigation system, thats where i would start.
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