I don't know if it matters for a Subaru, but I always apply a thin film of silicone grease or Sil-Glyde on the bushing i.d. so the swaybar won't squeak. Some manufacturers do so at the factory and it is mandatory if you upgrade to polyurethane bushings.
Like slacktide said, check the play in the sway bar end links since those are usually the culprit for knocking sounds.