I've lived in my house for almost 9 years. I recently had my septic tank emptied. I've noticed that about a few weeks after emptying the tank that whenever I flush or when the water is running in the bathrooms, I can hear the water running in my bedroom wall.
Here is the only plausible explanation I can think of. Obviously, there are pipes in the wall, they may not be carrying water, they may be vent pipes transmitting sound. Normally, the main line into the septic tank has its outlet below the waste water level. Now that the tank has been emptied the end is open to a large hollow box. This ends up being a sort of acoustic reverberation instrument that amplifies running water sounds. The sound travels back up the main drain and into the vent pipes in the wall where you can actually hear the amplified sound.
I predict once the tank fills and the pipe goes under water the sound will diminish again. You may still hear it slightly now that you know what to listen for, but it should be quieter than it is now.