  We present a study of current fluctuations in two models proposed as quantum
shuttles. Based on a numerical evaluation of the first three cumulants of the
full counting statistics we have recently shown that a giant enhancement of the
zero-frequency current noise in a single-dot quantum shuttle can be explained
in terms of a bistable switching between two current channels. By applying the
same method to a quantum shuttle consisting of a vibrating quantum dot array,
we show that the same mechanism is responsible for a giant enhancement of the
noise in this model, although arising from very different physics. The
interpretation is further supported by a numerical evaluation of the
finite-frequency noise. For both models we give numerical results for the
effective switching rates.
