I'm hearing running water sounds in my wall but I'm not running anything (no toilet flush, no faucets on, no sprinklers, etc.). When I turn off the water at the street, I still hear the sound but not as strong. It doesn't sound like water trickling like a leak. It's almost like a white noise sound. Just water running through pipes.

Any theories on what it might be? I'd hate for it to be a leak inside the walls or something.

It's on the exterior wall on the same side as my gas meter. Update: I can hear it through one of my interior walls as well.