A low-frequency oscillation (<100 Hz) has been observed in a low-pressure (1-50 mTorr) radio-frequency (RF) inductively coupled plasma, produced in sulfur hexafluoride. Langmuir probe studies have characterized this oscillation with respect to RF power, gas pressure and probe proximity to the antenna. The experimental parameter space within which this oscillation is observed is mapped with respect to power and pressure for the reaction chamber in use. The oscillation is observed in Langmuir probe currents for positive probe bias, and has a strong dependence on experimental conditions, as well as probe position within the chamber. The propagation speed of the instability away from the source is found to be 16 m s-1.