Try reducing the current to maybe 6 amps or so on the drives, and see if that helps? You could always try a different drive, like a Leadshine EM806 and see if it's smoother? I've heard that they are. Testing an Leadshine AM882 on a bench, I get super smooth rotation at even really slow speeds.
If you pull the motor, and run it on a table, does it still have resonance at the same speed?
I don't think it's a big deal, as it's unlikely you'll actually ever cut anything at 5ipm. But you'll have to see if it affects anything.