  The vibrational properties of ErTe$_3$ were investigated using Raman
spectroscopy and analyzed on the basis of peculiarities of the RTe$_3$ crystal
structure. Four Raman active modes for the undistorted structure, predicted by
factor-group analysis, are experimentally observed and assigned according to
diperiodic symmetry of the ErTe$_3$ layer. By analyzing temperature dependence
of the Raman mode energy and intensity we have provided the clear evidence that
all Raman modes, active in the normal phase, are coupled to the charge density
waves. In addition, new modes have been observed in the distorted state.
