In recent years, flat shape mufflers have been employed to save for space a trunk room and an indoor space. However, when flattening the muffler, the sound pressure level due to sound radiation from the muffler outer surface increases. We investigated sources of a flat shape muffler with s steady internal flow by using NAH method and measured vibration acceleration on the outer surface of the muffler and velocity fluctuation of the flow inside the muffler in order to identify the excitation source by the internal flow.