High gear mesh (number of teeth times running speed)
It is common to witness a peak at the gearmesh frequency. However, if the amplitude of the peak increases, or harmonics develop, or the sidebands of the input or output speed grow in amplitude, then it is highly likely that a fault is developing in the gearbox.
It is up to you what to do next. If you carefully analyze the spectrum and analyze a time waveform (that has been measured correctly) it is possible to pinpoint the fault condition.