Does this thing have a flapper valve? If not, then it must have some sort of valve that opens to let water go into the bowl to flush. My guess is that that is leaking. The noise you hear is probably the fill valve opening to refill the partially drained tank. Turn the water off before you go to bed. Check the tank level when you get up. My guess is that it is going to be lower than where you started. Then, you need for find and replace the worn part. Another way to do this is to put some food dye in the tank, and see if it ends up in the bowl. This won't work if you let it sit too long, as it could all flush down the drain. The tell is that the tank will be clear, though, if it is leaking, or there will be color in the bowl if it hasn't all been washed away.