  An X-ray imaging technique is used to probe the stability of 3-dimensional
granular packs in a slowly rotating drum. Well before the surface reaches the
avalanche angle, we observe intermittent plastic events associated with
collective rearrangements of the grains located in the vicinity of the free
surface. The energy released by these discrete events grows as the system
approaches the avalanche threshold. By testing various preparation methods, we
show that the pre-avalanche dynamics is not solely controlled by the difference
between the free surface inclination and the avalanche angle. As a consequence,
the measure of the pre-avalanche dynamics is unlikely to serve as a tool for
predicting macroscopic avalanches.
