Abstract: In response to the abnormal noise phenomenon in the EMU’s passenger room during the high-speed operation. Through the field test data, the sound source and the transmission of the vibration are analyzed, in both frequency and time-frequency domain. The result of data analysis shows that the abnormal noise of the passenger compartment is the acoustic radiation caused by the abnormal vibration of the floor. Compared with the bogie frame, the amplitude of the abnormal vibration on the axle box is relatively small, and the frequencies of abnormal vibration are the traction motor rotating frequency, along with its harmonic frequencies. Therefore, the possibility of abnormal vibration caused by the wheel-rail excitation and the equipment suspended on the car body is eliminated, and the fault is located at the transmission system. According to the vibration spectrum characteristics of the typical faults of the traction motor and the gearbox, it is found that they do not agree with the abnormal vibration frequency, therefore the fault can be locked at coupling. Based on the rotor dynamics theory, the force equations under the misalignment of the elastic coupling are established, and it suggests the misalignment effect will cause the rotor system to generate the harmonic frequencies vibration with the rotational frequency as the fundamental frequency, which is consistent with the analysis results. After replacing the coupling, the acceleration of the abnormal vibration on the passenger compartment floor is reduced, especially in idling condition. The root mean square values are reduced by 67%, and the peaks at 1X and 3X frequencies disappeared, the amplitudes of 2X and 4X are decreased by 78% and 66%，respectively. The vibration level of the floor of the passenger compartment achieved the ideal effect, and the theoretical analysis and test results have been mutually verified.

Key words: high-speed train, interior noise, multiple frequency vibration, fault diagnosis, coupling