Automobile sounds contain information on the technical state of the car: The example of the professionalization of the car mechanics craft during the interwar years shows how automobilists and mechanics experienced car sounds and how they tried to read the contained information. In a first step two listening practices will be distinguished: monitory listening and diagnostic listening. Then, a characterization of the listening practices will be given. The contemporary sources reveal that automobilists and mechanics equally appropriated the listening practices as social practice. Furthermore, the entanglement of the two listening practices, which were difficult to separate in everyday life, played an important role. In a second step it will be shown that the societal attribution of the listening practices to the two actor groups changed during the interwar years. In the struggle between automobilists and mechanics the latter got the upper hand and successfully claimed that they exclusively possessed the trained ear for the practice of diagnostic listening.