I have the Skip Frequency Parameters, what I meant was not all the cheaper Drives have these Parameters to set, but what you are seeing should not be happening at 50Hz it should be smooth and quiet just as it is at 20Hz
Your motor is very noisy at 50Hz it should be quiet at those settings, something is not set correct or the VFD is doing a really bad job, I don't think you would want a 6 pole motor, it would run too slow for your need
Swap the output wires around, it may run in reverse, of what you want, but sometimes this can have an effect on a motor driven by a VFD
Do you have good Grounding, motor to the VFD and Input power supply to the VFD