You are experiencing resonance in the stepper motor. This is normal and occurs at low speeds as you are observing.
To solve this you could use a stepper driver with an anti-resonance function built in such as the Geckodrive stepper motor controllers. If you’re current stepper driver hah anti-resonance, then it may need to be adjusted.
Another solution is to use a resonance damper on the back shaft of the motor. If you do a search you should be able to find a lot of information on the damper me.
Cheers
Peter.
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