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 can be suffering from gear and housing materials and also lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the mistake of over-specifying the electric motor. Remember, the input pinion on the planetary should be able handle the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the result stage must be strong enough to absorb the developed torque. Obviously, using a more powerful motor than necessary will require a bigger and more costly 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 protects 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 it’s impossible to totally eliminate noise from this 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 output shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive than lighter duty types. However, for fast acceleration and deceleration, a servo-grade gearhead could be the only wise choice. In such applications, the gearhead could be viewed as a mechanical springtime. The torsional deflection resulting from the spring action adds to backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate a number of construction features to reduce 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 type of bearings supporting the output shaft low backlash gearbox depends on the strain. High radial or axial loads usually necessitate rolling element bearings. Small planetaries can often get by with low-price sleeve bearings or other economical types with relatively low axial and radial load capability. For larger and servo-grade gearheads, durable result shaft bearings are often required.
Like most gears, planetaries make sound. And the quicker they operate, the louder they get.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are generally limited to about 50:1 and up, planetary gearheads lengthen from 3:1 (single stage) to 175:1 or even more, depending on the amount of stages.