Thanks for all the advice, this is an excellent forum, lots of
responders.

Here is what I have done so far:
(1) When I turn off the valve where the water enters the house, the
leak stops, so it is not in the front yard, but somewhere in or under
house.
(2) When i turn off the hot water heater, the leak is still there, so
it's in the cold water somewhere.
(3) I have turned off all water in the house, the sprinklers and the
outside faucets, and still have the leak.

Our house is on perimeter wall with about 2 ft crawl space. It's mushy
down there. It would really be nasty going down there
The leak is approximately 2-4 gal/hr

Our house was built in '89 so I think it is all copper down below.
Where would it most likely be leaking, in an elbow or a tee?
If it was leaking behind a fitting shouldn't the wall show some
dampness?