The environmental acoustic noise is considered as a big risk for today's population health. Consequently, the regulations in many countries commit themselves to control the exposition of people, imposing limits to the noise level. In the comparison between the measured value and the threshold, the uncertainty of the measured value has to be taken into account. In this paper, a procedure for the evaluation of the uncertainty of traffic noise measurements due to the variability of the measurand is proposed. In particular, after possible outliers have been identified and removed, then the uncertainty is estimated on the `purified' signals by using the bootstrap-t method.