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 out more for quieter, smoother gears.
Don’t make the error of over-specifying the 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 output stage must be strong enough to absorb the developed torque. Certainly, using a more powerful motor than necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, output torque is a linear function of current. Therefore besides protecting the gearbox, current limiting also defends the electric motor and drive by clipping peak torque, which can be 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 remove noise from this assembly, there are several ways to reduce it.

As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Thus the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead could be viewed as a mechanical spring. The torsional deflection caused by the spring action increases backlash, compounding the consequences of free shaft movement.
Servo-grade gearheads incorporate many construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter output shafts and low backlash gearbox beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads have a tendency to be the costliest of planetaries.
The kind of bearings supporting the output shaft depends on the strain. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often get by with low-cost sleeve bearings or various other economical types with relatively low axial and radial load capability. For bigger and servo-grade gearheads, heavy duty output shaft bearings are usually required.
Like most gears, planetaries make sound. And the quicker they run, the louder they get.

Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are usually limited by about 50:1 and up, planetary gearheads prolong from 3:1 (single stage) to 175:1 or more, depending on the number of stages.