During the December 2005–January 2006 non-eruptive period, the tremor amplitude at Etna markedly increased and negatively correlated with the gravity signal from one of the two summit station, over 2–3 hour periods. No correlation was found with the signal from the other gravity station. We locate the tremor source by inverting the spatial distribution of seismic amplitudes. Relying on the relative position of the two stations, we define a volume within which the gravity source must lie. During the period of marked anti-correlation, the tremor sources intersect this volume in a region located 1 km S-SE of the summit craters and about 2 km beneath the surface. This finding suggests that the anti-correlation marks the activation of a joint source process, possibly related to the arrival of fresh magma and the consequent gas separation. Our study has implications for the early recognition of gas segregation processes at active volcanoes.