In order to improve sound quality of vehicle noise, the idling exterior noise of a small diesel engine truck and the interior noise of a small-size sedan were varied artificially using a digital signal-processing technique in the frequency domain. First, the original exterior idling noise and its variations were tested using a paired-comparison technique concerning their unpleasantness. As a result, the reduction of the high-frequency component, i.e., -6 dBA compared with the original, was most effective in reducing unpleasantness of exterior idling noise. To realize this effect in a very small diesel engine truck, an engine compartment enclosure was fixed to the original truck. The result was quite successful in achieving a significant reduction in the high-frequency noise component. Second, interior noises of a small size sedan with four cylinders and a four-cycle engine were varied in their second-order component of engine revolutions and their booming sensation was tested using magnitude estimation and categorical judgment in seven categories in the requirement of improvement. It was clarified that the reduction of the second-order component gave a significant reduction in booming sensation. In addition to the subjective experiments, the relation between the subjective results and the psychoacoustical measures was also considered.