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Taking a sixcylinder diesel engine as the example, noise signals were recorded in different working conditions. Based on the wavelet packet decomposition and coefficient reconstruction of the recorded noise signals, signals in various frequency bands were extracted. Subband signals relevant to the engine combustion process were combined and reconstructed, and noise signals resulted from the combustion excitation were obtained. Moreover, the continuous complex wavelet transform was adopted to analyze the reconstructed signal. The three-dimensional wavelet power spectrum was also calculated to investigate the combustion condition of each cylinder and extract the related information. The analysis results show that the signal energy of combustion noise accounting for small proportion mainly distributes in the high frequency region of 20.480～24.576 kHz, when engine works at the speed of 2200 r/min with full load. At the idle condition of 650 r/min, combustion noise signal energy is mainly concentrated in the medium and low frequency band of 1024～2521 Hz, and has a larger contribution to engine noise.