I would not think that this difference is cause for great concern, this may be due to slight differences in motor/drive characteristics, but if the ringing just started, it may be something has changed in the drive and a sign that future problems may appear.
Many VFD's have the ability to change the switching frequency, sometime bumping it up to a higher value gets rid of the ring.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.