Perhaps the most obvious is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the guts distance of the tooth mesh. Sound can be affected by gear and housing components as well as lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the input pinion on the planetary must be able handle the motor’s result torque. Also, if you’re utilizing a multi-stage gearhead, the result stage should be strong enough to absorb the developed torque. Obviously, using a more powerful motor than necessary will require a bigger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque can be a linear function of current. Therefore besides protecting the gearbox, current limiting also shields the motor and drive by clipping peak torque, which may be from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are simultaneously in mesh. Although it’s impossible to totally get rid of noise from this assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Therefore the gearhead can be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are generally more costly than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In this kind of applications, the gearhead could be seen as a mechanical springtime. The torsional deflection resulting from the spring action adds to backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate many construction features to minimize torsional stress and deflection. Among the more common are large diameter result shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads tend to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends on the strain. High radial or axial loads usually necessitate rolling element bearings. Small 
planetaries can often get by with low-cost sleeve bearings or other economical types with relatively low axial and radial load ability. For bigger and servo-grade gearheads, heavy duty output shaft bearings are often required.
Like most gears, planetaries make noise. And the faster they operate, the louder they get.
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