Fault detection using a vibration signal is considered to be one of the most important methods applied to rotating parts. Most of the rotating machine faults can be detected by this technique before dangerous situations occur. Although the vibration signal is easy to be measured and recorded, its analysis is by no means simple. Moreover, when applying this technique to a real object in the field, e.g. to a gearbox used in a military vehicle including main and transfer gearboxes, the vibration signals are modulated a lot. There are too many factors affecting the vibration signal and too many components that can generate a vibration, therefore the vibration analysis is usually difficult.