What causes avoidable noise?

You answered correctly. Avoidable noise is caused, for example, by using the horn just to greet someone and by slamming car doors shut forcefully. Both actions are unnecessary and create noise that disturbs others without any valid reason. According to German traffic regulations, unnecessary noise should be avoided to protect the environment and public peace. Driving with a roof-rack, however, is not considered avoidable noise since the noise from a properly installed roof-rack is generally accepted as part of normal vehicle use. Being aware of what constitutes avoidable noise helps you act considerately and within the rules while driving in Germany.