How do I find the source of a leak?

Actually, I'm not sure there is a leak. I've been hearing the sound of running water behind a wall in my bathroom for three weeks. Three plumbers could not find the source and want to break down walls to locate it. There is no evidence of of leak, no moisture on the wall, no water on the floor, no faucets leaking. I finally turned the water off to the whole house and there is no sound at all in the wall. Here's my stupid question: If the water is "leaking" under the slab, would I hear it in the walls, and would it stop abruptly just by turning the water off? I don't want to tear down walls if I don't have to. I need guidance.