That sounds like a broken bearing. Maybe one of the spheres is broken. You might need to replace the linear rail slider. Maybe such videos are a help here:
I was not talking about the height above the workpiece, I was referring to the position of the head on the rail. I quickly drew an image:
The deeper the head is mounted, the higher the lateral/bending forces at the slider of the rail. Especially at high speeds. That’s why I suspect the ball bearings could have been damaged in your setup. Try to have the head mounted as high as possible during fast operation. If your base is too low, you need to raise your workpiece (the focus distance should remain the same).