An internal active vibration control system for suppressing gearbox housing vibrations due to gear transmission error excitation is developed and evaluated experimentally. The approach is based on an active shaft transverse vibration control concept that resides within the gearbox. The system contains a piezoelectric stack actuator for applying control forces to the shaft via a secondary rolling element bearing. An enhanced filtered-x least mean squares control algorithm with frequency estimation capability is employed to generate the appropriate actuation signal. The experimental results show up to about 20 dB reduction in the vibration levels of the first two mesh harmonics at the control positions on the housing structure for a range of operating speeds. The experimental studies also indicate that, under certain narrow operating conditions, the vibrations at some non-control positions may be amplified slightly due possibly to the effect of unmodeled dynamics. In spite of this limitation, the proposed active vibration control approach is quite promising for reducing gear vibration and ultimately gear noise.