We analyze time series of Strombolian volcanic tremor recorded with a network of broadband seismic stations. We evidence the occurrence of a significant component of volcanic tremor in the range <0.2--0.5> Hz by using linear and nonlinear methods. The polarization and ZLCC analysis show that the wave field is characterized by high apparent velocity (~2 km/s). It comes mainly from the crater area with a clear amplitude decay towards the sea level. The previous properties are loosed when the tremor level is higher suggesting that in this case the wave field is more affected by oceanic noise. The waveform extracted by Independent Component Analysis (ICA) shows a behaviour like nonlinear beats as in the case of high frequency waveforms. We conclude that this component displays a behaviour like the wave field at higher frequencies in spite of its low frequency, and it is produced by the vibrations of the volcanic conduit.