99 times out of 100 you don't need a new knuckle and would be a complete waste of money. If you're saying there is play in the rear wheel that you think is making noise I'd say go with a new knuckle. I had play in mine years ago and put in a new bearing. Didn't have noise just had a little play. When the new bearing was in the knuckle I still had play. Later down the road I put on a new knuckle and problem was fixed.