That's an interesting question. I feel like I'm in college taking an English test! I think that Lewis was alluding to the chains that Orual saw her sister Psyche bound with at the ShadowBrute (the god Eros') mountain. The chains are a constant reminder of Psyche's bond to Orual and her sister's fate after trying to shine a light in the face of her lover. She is banished to walk the earth in poverty and chains. The sound of chains on the wind is the reminder to Orual that her sister is out there, somewhere. It is also just a bit creepy, don't you think? I think C.S. Lewis builds wonderful dramatic tension in the story and shows how haunted Orual is by her decision to interfere or save her sister, depending on how you interpret her actions.