  We study the emergence of density waves in dipolar Bose-Einstein condensates
(BEC) when the strength of dipole-dipole atomic interactions is periodically
varied in time. The proposed theoretical model, based on the evolution of small
perturbations of the background density, allows to compute the growth rate of
instability (gain factor) for arbitrary set of input parameters, thus to
identify the regions of instability against density waves. We find that among
other modes of the system the roton mode is most effectively excited due to the
contribution of sub-harmonics of the excitation frequency. The frequency of
temporal oscillations of emerging density waves coincides with the half of the
driving frequency, this being the hallmark of the parametric resonance, is
characteristic to Faraday waves. The possibility to create density waves in
dipolar BECs, which can persist after the emergence, has been demonstrated. The
existence of a stationary spatially periodic solution of the nonlocal
Gross-Pitaevskii equation has been discussed. The effect of three-body atomic
interactions, which is relevant to condensates with increased density, upon the
properties of emerging waves has been analyzed too. Significant modification of
the condensate's excitation spectrum owing to three-body effects is shown.
