As Q factor is boosted in microscale optical resonant systems there will be a natural tendency for these systems to experience a radiation-pressure induced instability. The instability is manifested as a regenerative oscillation (at radio frequencies) of the mechanical modes of the microcavity. The first observation of this radiation-pressure-induced instability is reported here. Embodied here within a microscale, chip-based device this mechanism can benefit both research into macroscale quantum mechanical phenomena  and improve the understanding of the mechanism within the context of LIGO . It also suggests that new technologies are possible which will leverage the phenomenon within photonics.