The DRI-DWW Noise Abatement Programme is a joint cooperation between the Road and Hydraulic Engineering Institute (DWW) in the Netherlands and the Danish Road Institute (DRI) within the framework of the Dutch Noise Innovation Programme (the IPG programme). The present report addresses the effect of cleaning a porous pavement on A28 at Staphorst. Cleaning was performed by three different contractors. From the assumption that cleaning reduces the amount of mastic and increases the voids content the effect of cleaning has been assessed by CT-scanning of drill cores. Differences between pairs of drill cores before and after cleaning are assessed from the vertical profiles of voids or mastic content and CT-scan images. From all profiles of voids and mastic contents it is noted that in general cleaning has very limited or almost no effect on clogging. Only in specific positions differences between voids or mastic content before and after cleaning are observed. Though, this might be due to differences in voids or mastic content between the two positions in the pavement before cleaning. Only if differences in voids content are confirmed by similar differences in mastic content the differences is attributed to the effect of cleaning.