Well, it looks like my tower is doing it too, but I just never hear it over the roar of the fans. However, it's doing it with significantly less frequency. It seems to revive update a few times after left idling, but then it appears to stop.
I found one person with the same issue in
this thread over at MacOSXHints.com.
I'm going to post to Apple's own support area to see if I can find some answers.