  We consider here scalar and electromagnetic perturbations for the Vaidya
metric in double-null coordinates. Such an approach allows one to go a step
further in the analysis of quasinormal modes for time-dependent spacetimes.
Some recent results are refined, and a new non-stationary behavior
corresponding to some sort of inertia for quasinormal modes is identified. Our
conclusions can enlighten some aspects of the wave scattering by black holes
undergoing some mass accretion processes.
