The excitation mechanism of low degree acoustic modes is investigated through the analysis of the stochastic time variations of their energy. The correlation between the energies of two different modes is interpreted as the signature of the occurrence rate of their excitation source. The different correlations determined by Foglizzo et al. (1998) constrain the physical properties of an hypothetical source of excitation, which would act in addition to the classical excitation by the turbulent convection. Particular attention is drawn to the effect of coronal mass ejections. The variability of their occurrence rate with the solar cycle could account for the variation of the correlation between IPHIR and GOLF data. Such an interpretation would suggest that the mean correlation between low degree acoustic modes is at least 0.2 % at solar minimum.