The rear drive portion of the scooter, usually referred to as the drive train or "transaxle", contains the motor, gearbox and magnetic braking mechanism. The scooter manufacturers tell us that these drive trains often make a wide ranges of sounds, that often vary from scooter to scooter even when the models are identical. Sounds can range from a whining, whirring, clunking, clicking, humming, etc.
Sounds can change as time passes and can be more pronounced if the rider/cargo weight is approaching the maximum weight capacity of the scooter.
All of these sounds are normal and if the scooter is operating properly in all other regards, nothing need or can be done to change the sound. None of these sounds are indicators of a defect, failure or coming failure.