We have studied, by dark-field electron microscopy, the melting of lead clusters of about 200 Å in size embedded in a SiO matrix. Structural changes are observed before the melting temperature Tm is reached. At low temperatures (T < 290 C) their frequency is temperature independent and seems to be dominated by the intensity of the electron beam. Close to Tm the frequency becomes strongly temperature dependent. The data suggest that it diverges at Tm. This behaviour is interpreted as indicative of activated rotation of the clusters in the SiO host matrix prior to bulk melting.