  We present the results of a short time scale monitoring of SiO maser emission
(v=1 J=1-0 transition) in four known strong sources. These sources were
monitored nightly for a period of about a month. The aim of these observations
is to investigate the possible presence of variations in the maser lines on
time scales of a few days to weeks, due to sound waves propagating out from the
central star. If sound waves are responsible for the mass loss of certain cool
giants, as suggested by Pijpers and Hearn (1989) and Pijpers and Habing (1989),
local variations in density and relative velocity are expected just above the
stellar photosphere. These could give rise to variations in any narrow spectral
line formed in this region, and therefore in particular in the SiO maser lines.
Our observations indicate that variations in the line shape (leading to
relative changes in the intensity of about 20\%) occur in the SiO emission of
Mira type stars, within short time scales of 10-20 days. The main component of
the profile variability is consistent with a displacement of the velocity
centroid of the dominant maser peaks, by about 1 km s$^{-1}$ in the average.
Apparent variations in the total line flux were also found, but could be
partially due to calibration uncertainties.
