Perhaps the most obvious is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound is also 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 mistake of over-specifying the engine. Remember, the input pinion on the planetary should be able 
handle the motor’s result torque. Also, if you’re using a multi-stage gearhead, the output stage should be strong enough to absorb the developed torque. Obviously, using a better motor than required will require a larger and more costly gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque is definitely a linear function of current. Therefore besides safeguarding the gearbox, current limiting also shields the motor and drive by clipping peak torque, which can be anywhere from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are concurrently in mesh. Although you can’t really totally get rid of noise from such an assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are generally more expensive than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead could be the only wise choice. In this kind of applications, the gearhead may be viewed as a mechanical springtime. The torsional deflection caused by the spring action increases backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate several construction features to minimize torsional stress and deflection. Among the more prevalent 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 type of bearings supporting the output shaft depends on the load. High radial or axial loads generally necessitate rolling component bearings. Small planetaries could get by with low-price sleeve bearings or various other economical types with fairly low axial and radial load capability. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like the majority of gears, planetaries make noise. And the low backlash planetary gearbox faster they operate, the louder they get.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are usually limited to about 50:1 or more, planetary gearheads expand from 3:1 (single stage) to 175:1 or more, depending on the amount of stages.