This work describes the noise diagnosis of a four-stroke engine by using the silenced Schenck NT 100 dynamometer in the engine semi-anechoic chamber, which is a new test facility of ARTC. The study was performed for identifying an unpleasant noise buried at engine speed 5000 rpm by using B&K sound quality evaluation system to record, analyze, edit and playback the noise. The results is shown that the unpleasant engine noise was contributed from the 8-th harmonic of engine speed. With the assistance of spatial transformation of sound field technique, the location of noise radiation and its generation mechanism were found. The noise was deemed to be caused by the coincidence of excitation force and the CVT case structure resonance at 668 Hz.