On 1996 March 7, the Solar and Heliospheric Observatory spacecraft conducted a multi-instrument campaign to observe polar plumes in the south polar coronal hole. Recent time-domain analyses of EUV Imaging Telescope images from that campaign show filamentary substructure in the plumes, on a length scale of ~5'', which changes on timescales of a few minutes, and coherent quasi-periodic perturbations in the brightness of Fe IX and Fe X line emission at 171 Å from the plumes. The perturbations amount to 10%-20% of the plumes' overall intensity and propagate outward at 75-150 km s-1, taking the form of wave trains with periods of 10-15 minutes and envelopes of several cycles. We conclude that the perturbations are compressive waves (such as sound waves or slow-mode magnetosonic waves) propagating along the plumes. Assuming that the waves are sonic yields a mechanical energy flux of 150-400 W m-2 (1.5-4 ×105 ergs cm-2 s-1) in the plumes.