The sound is not one of running water. But it's difficult to describe.
Based on a similar thead in another forum, the only clue I've got as to what it might be is the sound of the system represurizing after an irrigation valve opens (i.e. water on far side of valve is at 0 psi, so that none-pressure "leaks" back into the system when the valve opens).
Now for what it's worth, I can tell you that I designed this system such that nearly all the water remains in the irrigation lines when the valve shuts off, so I don't get a lot of air pushed out the pipes when the system cuts on, and perhaps my system is repressurizing quicker than a system that is allowed the pipes to drain.