Hhhmm
Yep! Really sounds like noise.
Are you using single ended A/B channel encoders or Differential A/A-/B/B- channel encoders ?
Single ended are very prone to noise, best to swap out for differential ended if you can. Make sure that you have the the cables shielded at least at the amplifier and that the cables are not running next to that 480V supply.
intrustion