This paper took the engine side cover as an example to study and identified the transmission noise and radiation noise by experiment and simulation analysis method. Firstly, sound signal of the engine side cover was collected by means of microphone arrays testing equipment, the signal spectrum was analyzed by harmonic analysis. The collected acceleration signal beside the bolt of engine side cover was taken as the excitation to calculate the frequency response of engine side cover. The surface vibration data of the engine side cover were imported to the software Virtual. Lab as the input loads of the boundary element model. The radiated surface acoustic power was calculated by boundary element method. The sound transmission loss was calculated by structural acoustic coupling model of the side cover. The results show that the noise between 20-8000Hz and 2500-3000Hz are mainly transmission noise, and 800-2500Hz is mainly radiation noise. This paper provides a new way for engine noise source identification.