Corrupt sectors of hard drive

If it's making that noise, the disk is probably screwed up physically. Don't store anything on it that you can't afford to lose. There are software tools that will scan the disk surface for errors, and mark any bad sectors to prevent files from being stored there, hence preventing file corruption. I don't know much about OS X, but this appears to be one such tool: http://docs.info.apple.com/article.html?.../mh12.html