Gear meshing produces a certain sound; during the first hours of running it may be a little more noticeable. What matters is the character of the sound and whether it changes over time.

SoundPossible cause
Continuous, even humNormal gear noise; may vary slightly with load
Rustling or growling that rises with speedBearing damage or lack of oil
A click or knock once per revolutionDamaged tooth, shaft runout, loose pulley or key
Knock at start and stopGear backlash, worn coupling, loose feet or torque arm
Noise increasing over timeWear, oil degradation, overload

First checks: oil level, gearbox and motor fixing bolts, proper seating of the motor shaft in the input bore, coupling or pulley alignment, and the load on the output. To isolate the source, the motor can be separated from the gearbox and run alone if the connection allows. Motor-side bearing and vibration topics are covered in the motor maintenance and overheating questions guide.