Y axis moves forwards and back when X moves left to right (Vcore 3.1 500mm)

When my toolhead is moving left and right in big moves I can see visible shaking of the Y axis. It moves about 1mm forward and back about 5 times during each side to side movement.

No idea how to troubleshoot and until then I have a $1600 paperweight I can't even fit out my door.
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