Perhaps the most obvious is to improve 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 and also lubricants. In general, be prepared 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. What’s more, if you’re utilizing a multi-stage gearhead, the result stage must be strong enough to soak up the developed torque. Obviously, using a more low backlash planetary gearbox powerful motor than required will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is definitely a linear function of current. So besides protecting the gearbox, current limiting also defends the 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 simultaneously in mesh. Although it’s impossible to totally eliminate noise from such an assembly, there are many methods to reduce it.

As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Therefore the gearhead could be close in diameter to the servomotor, with the result 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 sensible choice. In this kind of applications, the gearhead may be seen as a mechanical spring. The torsional deflection resulting from the spring action increases backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate several construction features to minimize torsional stress and deflection. Among the more common are large diameter output shafts and beefed up support for satellite-gear 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 generally necessitate rolling element bearings. Small planetaries can often get by with low-cost sleeve bearings or additional economical types with relatively low axial and radial load capacity. For larger and servo-grade gearheads, heavy duty output shaft bearings are usually required.
Like most gears, planetaries make noise. And the faster they operate, the louder they get.

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