  We study the fluctuation-induced, time-dependent force between two plates
immersed in a fluid driven out of equilibrium mechanically by harmonic
vibrations of one of the plates. Considering a simple Langevin dynamics for the
fluid, we explicitly calculate the fluctuation-induced force acting on the
plate at rest. The time-dependence of this force is characterized by a positive
lag time with respect to the driving, indicating a finite speed of propagation
of stress through the medium, reminiscent of waves. We obtain two distinctive
contributions to the force, where one may be understood as directly emerging
from the corresponding force in the static case, while the other is related to
resonant dissipation in the cavity between the plates.
