Water going on and off inside the walls

I keep hearing water going on and off inside the walls. No one is flushing toilets or running the water. I have a septic system. What is causing this?

I would check for leaking flappers on your toilets.

The tanks may be refilling.

What Terry said. One way to check this. as long as your not sing any drop in bowl cleaners that changes the color of your water. Just before you go to bed add food coloring to the tank of water in the water closet. DO NOT FLUSH. let it sit over night and in the morning give a look if you see the food coloring in the bowl that means the flapper is leaking and needs to be replaced.

Even easier is to close the stop valves on all your toilets and see if the noise goes away. If it does not, check your water meter to see if it is spinning.