By my experience a bad wheel bearing will have a lower-pitched moaning that manifests itself at higher speeds and when turning in a particular direction. For example, my right-front bearing was bad, so I could hear it when turning left because the weight of the car shifts to the right side. The sound was noticeable at about 35mph, and got really loud at 70mph.
Does any of this sound like what you're experiencing? I suppose if the bearing is really shot you might start hearing it all the time, but I don't know if it would still moan, or if it would sound like the scraping you describe.
