Reduction of alternator noise has become an increasingly important task in automotive industry as requirements for passenger compartment comfort increase, and other components such as the engine, exhaust system, etc. are made quieter. However, the aim of this study is to identify the noise sources of a numbers of alternator bands and to develop design guidelines to reduce the overall noise levels sacrificing the specified performance. With these guidelines, design engineers can reduce alternator noise effectively.To identify mechanical, aerodynamic and electromagnetic noise generation mechanisms, the alternator was reconfigured in many different ways, so as to separate each individual noise source. Noise generated by each configuration running at different speeds was measured, and its characteristics were analyzed. A special test stand was designed and fabricated, and tests were conducted in the automotive laboratory. To accomplish these goals, the diagnosis of the alternator noise is conducted in two cases, i.e., with load and without load (unloaded). In the case of unloaded, the alternator mechanical and aerodynamic noises can be identified, while in the case of with load, focus is made on alternator electromagnetic noise generation mechanism under various loading conditions and at different speeds. The results indicate that the noise levels of alternator increase with speed, but are nearly independent of loading conditions particularly at relatively higher speed. In other words, at speed range used in this study, the noise levels is a combination of aerodynamic, mechanical and electromagnetic noises, where the mechanical noise is predominant (strength ratio is between 60 - 70%) followed by aerodynamic noise (strength ratio is between 20 - 30%) with electromagnetic noise generation has the lowest level (strength ratio is 10%).