From the analysis of radial velocity measurements of the Sun, obtained at Izaña during long observing seasons covering ten years, from 1976 to 1985, stable periods longer than 1 day, have been found in the observed signal. In particular the appearance of an oscillation with a 13 day period, discovered by Claverie et al (1982), has been confirmed. The comparison, for 1981 - 82 - 83 and 84, of the observations with a calibrated numerical model of the passage of inhomogeneities (spots and plages) on the solar surface, shows that the signal is not only due to this effect. The obtained signal from the model has a phase lag of almost two days relative to the observed one. So, it is believed that another velocity field, probably related to the surface inhomogeneities, contributes to the observed signal.