Perhaps the most obvious is to increase precision, which really is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound can be affected by gear and housing components and also low backlash gearbox lubricants. In general, be prepared to spend more for quieter, smoother gears.
Don’t make the mistake of over-specifying the motor. Remember, the input pinion on the planetary must be able manage the motor’s result torque. What’s more, if you’re using a multi-stage gearhead, the output stage should be strong enough to soak up the developed torque. Certainly, using a more powerful motor than necessary will require a larger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque is definitely a linear function of current. So besides protecting the gearbox, current limiting also protects the 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 simultaneously in mesh. Although you can’t really totally get rid of noise from this assembly, there are several methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits the form of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are usually more costly than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In such applications, the gearhead could be viewed as a mechanical springtime. The torsional deflection resulting from the spring action increases backlash, compounding the effects of free shaft motion.
Servo-grade gearheads incorporate many construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter result shafts and beefed up support for satellite-gear shafts. Stiff or “rigid” gearheads have a tendency 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 can often get by with low-cost 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 usually required.
Like most gears, planetaries make noise. And the quicker they run, the louder they get.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are generally limited to about 50:1 and up, planetary gearheads prolong from 3:1 (one stage) to 175:1 or more, depending on the number of stages.