When a creaking sound is coming from the rear wheel, then the culprit is often the connection between freehub body and cassette. To solve this, remove the cassette, clean and re-install. When re-installing the cassette, apply a small amount of grease to the freehub body. If there are still noises coming from this area, then the freehub is most likely the problem. To fix this, you would need to take the entire thing apart and because of a freehub’s complexity, we do not recommend doing this yourself.