pwm drive
As a rule-of-thumb I'd say that the typical PWM frequency for DC servo drives is about 10...20 kHz, at least in the DIY world. Perhaps it'd be best to use an adjustable PWM frequency, so you can check when the frequency is too high for the application. Usually too high a frequency results in excessive heating of the motor due to losses.
IMO the most annoying thing with these kind of drives is the high-pitch whine due to too low a PWM frequency, but many people have their hearing already limited to around 10 kHz or something - depending on age, personal aging and possible long-term exposure to various loud noise. Your mileage may vary.
What kind of PWM drive do you have? A self-made one or a commercial one? Some simple drives use just a triangle-wave generator with a couple of comparators to make the PWM. In that case, the triangle-wave frequency might be possible to adjust.
Hobbie