Volcanic activity at the summit craters of Mt. Etna includes phreatic eruptions which jeopardize tourists and other people who sporadically approach the top of the volcano. To date no appreciable precursors of this kind of activity are known. Solely intermittent volcanic tremor might be considered a short-term seismic forerunner. The pattern of volcanic tremor amplitude at Mt. Etna during springtime 1987 was investigated and revealed significant periodicities before both phreatic explosions occurring at the summit craters during that time span. Results are in agreement with a boiling mechanism as the source of intermittent volcanic tremor, even if the lack of data on the ground water table in the upper part of the volcano did not allow the formulation of quantitative models.