I am working on a Comdial DX-80 and the CO lines have digitalish noise on them.

Extension to extension is fine, Extension to VM is fine. All extensions hear noise on CO lines.

(4) lines, (2) 8-port extension boards.

Noise on COs from both stations boards.

Since the CO lines are on the motherboard, would a CO board solve the issue do you think or would the motherboard or bus continue with the line noise?

Basically, if you press a CO line and press a digit so you can hear it better, the lines have a digitally scratchy, whiny rattling noise on them.