My in-laws are hearing running water when inside their shower. I
turned off the main and bled the lines and it stops. When the main is
back on, the sound is back. So I'm sure it is a water leak either in
the wall or under the slab. Unfortunately, both sides of the wall
where the sound is coming from are tiled. If I remove the shower
faucet, that will give me a small access to the inside of the wall,
but it is too small to see very well. Is there any way I can probe
down to determine if the leak is in the copper ging from the slab up?

What I want to do is make certain it is an under slab leak before I
have someone come out and re plumb the entire house.

Thanks in advance.