Perhaps the most obvious is to improve 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 in addition to lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the mistake of over-specifying the electric motor. Remember, the input pinion on the planetary must be able deal with the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the result stage should be strong enough to soak up the developed torque. Obviously, using a better motor than required will require a bigger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque can be a linear function of current. So besides safeguarding the gearbox, current limiting also defends the electric motor and drive by clipping peak torque, which may 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 remove noise from this assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Hence the gearhead could be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are usually more costly than low backlash gearbox lighter duty types. However, for speedy acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In this kind of applications, the gearhead could be seen as a mechanical spring. The torsional deflection resulting from the spring action adds to backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate many construction features to minimize torsional stress and deflection. Among the more prevalent are large diameter output shafts and 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 upon the load. High radial or axial loads usually necessitate rolling element bearings. Small planetaries can often 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, heavy duty result shaft bearings are often required.
Like most gears, planetaries make noise. And the faster 
they run, the louder they obtain.
Low-backlash planetary gears are also available in lower ratios. Although some types of gears are usually limited by about 50:1 or more, planetary gearheads extend from 3:1 (solitary stage) to 175:1 or more, depending on the number of stages.