If you leave the water on for say 45 minutes,you will find a wet spot if you look hard enough,if there is one that is.It would be a tiny leak if it took 45 minutes,it would take 20 seconds if it was a good leak. What you are most likely hearing is a cavatation type sound.somtimes the valves dont fully open as they age,there is probably a couple other causes like multiple elbows too.Air and water bounce around in the pipe and vibrate and can have a wide range of sounds. It can be quiet enough where you wouldnt really hear it,or last week i heard one that sounded exactly like a helicopter,extremely loud and it would wake everyone in the house up. this is most likely the problem. you might just have to dig a little more to find the problem if it is not cavitation.
a stethoscope might help to narrow down location.other than that probably just have to dig both ways until you find it. and possibly put a small section of new pipe in there,if it is near a tree, roots will kink the pipe right up,this could also be your problem.good luck