A unique experiment has been conducted to identify the dependence of variations in truck noise emission measurement data upon changes in vehicle performance and sound propagation properties. A comparison of data measured at different sites and at different times at the same site by previous investigators [e.g., J.E. Piercy et al., “Mechanisms Causing Variability in Noise Testing of Light Motor Vehicles,” NRC, Ottawa, Ontario (1976)], has shown a variability in the mean maximum A-weighted sound pressure level of up to 4 dB. Little similar data is available for medium and heavy-duty trucks. In this experiment, preliminary data have been obtained for one gasoline and two diesel-fueled trucks performing SAE J366b drivebys, simulated drivebys, and idle-maximum idle runups. The results obtained to date, as part of a one year series of tests, are presented as a function of several of the various performance and environmental variables.