Artificial exterior diesel idling noise and its variations with varying its parameters such as frequency spectrum and time wave form were made using the generation techniques developed in the author's laboratory. The subjective evaluations on unpleasantness of the artificial idling noise and its variations were conducted through the paired comparison technique. As a result, the reduction of sound pressure level of high frequency component was the most effective countermeasure for better sound quality with less unpleasantness and this was realized by engine compartment enclosure in the real small diesel engine truck. The result was quite successful. Togather with this result, psychoacoustic scalings such as loudness, roughness, fluctuation strength and so on were tested for estimating unpleasantness of the exterior idling noise.