It does make sound, just because there are no receptors does not mean that an object does not make sound. For example, whales communicate ona frequency higher then our ears can hear, however, we know that they still make sound. Another example is thunder; If you were far away to see thunder but not hear it, does it make sound? It does. Objects make sound by vibrating, there is no connection to sound and the presence of a receptor.
The Net Force of an object should be the difference between the forces applied to the object if the object is not in equilibrium. This object is not in equilibrium so therefore by finding the difference between the forces, you will find your answer. 20 N - 7 N = 13 N.
