I rebuilt my keyboard this week and everything sounds good except the spacebar. The stabilizers are lubricated, but there is still a light rattle on the left side. Would you check the wire balance first or add a small amount of dampening inside the keycap?
I would check the wire on a flat surface before adding anything to the keycap. I had one that looked fine but lifted slightly at one end, and correcting that removed most of the rattle.
Also make sure the keycap is not slightly warped. If another spacebar fits the same stabilizers without rattling, the cap may be the whole problem.