What causes avoidable noise?

Your answer is correct because both sounding the horn as a greeting and firmly slamming doors cause avoidable noise according to German traffic rules. The horn should only be used to warn others of danger, not for non-urgent communication like greetings. Similarly, slamming car doors loudly creates unnecessary noise that disturbs the environment. Using a roof-rack, however, is not considered avoidable noise. Understanding these distinctions helps reduce noise pollution and promotes considerate behavior in traffic and parking areas.