This paper presents the results of recent investigations carried out at Isuzu Motors Ltd. into the reduction of noise radiated from exhaust system of heavy duty diesel truck. Radiation noise from the exhaust system is considered to be originated from the following vibrations. One is the vibration transmitted from exhaust manifold and the other is the vibration excited by fluctuation of inside gas pressure. Each contribution to the noise is analysed in conjunction with experimental results. Also presented are the results of a study into the optimum way of isolating the vibration and the practical way of damping by using laser holographic interferometry technique.