We conducted a series of electrokinetic measurements on partially saturated sandstone samples. Both steady and transient electric signals were observed. We recorded transient electric signals with frequencies of 0.5--2Hz and amplitudes 10--100 times smaller than the quasi-DC streaming potential, which we associate with fluid flow instabilities in the unsaturated porous rocks samples. Some of these electric signals were correlated with fluid pressure variations, delayed by one to a few seconds.