I also have this. My Classic is 2 days old. I was semi-concerned about this as well, but from all the "me too" posts on here, I think it's safe to say it's normal hard drive noise.
It's usually a non-issue, because when wearing headphones, I don't hear it. I only hear it when I have the Classic hooked up to speakers when watching a movie or TV show on it, and it's definitely audible from a couple feet away, and happens while a movie is playing now and then.
But like I said, after reading thru this thread, I'm officially not concerned about it.