For the noise problem of an automotive claw pole alternator, acoustic testing is conducted to investigate its noise sources. Firstly, the vibration and noise of four different prototypes are measured under no-load and load conditions. Then, the mechanical noise, aerodynamic noise and electromagnetic noise are identified based on order analysis. The generation mechanism of aerodynamic noise and electromagnetic noise is revealed through the flow field simulation and electromagnetic field theory. Finally, the contribution of each noise source is analyzed. The results show that electromagnetic force would cause 6k(k=1,2,3…) order electromagnetic noise. Cooling fans, rotor and slotted stator would generate aerodynamic noise, among them front fan produces the integer order (11st, 12nd, 13rd,…) noise, rear fan produces the even order (8th, 10th, 12nd,…) noise, rotor produces 6k (k=1,2,3,…) order and stator produces 36th order. At low and middle speed, noise is mainly from 36th order electromagnetic noise while at high speed, the aerodynamic noise (8th, 10th and 12nd order) is dominant. As the mechanical noise is low, it has little effect on the overall noise.