Why does my external hard-drive constantly vibrate?

I have an Iomega external hard-drive (model = LDHD250-U), which constantly vibrates while it is powered on. I run backups overnight, which is the only time that I access the drive. Most of the time, it should just sit idle, except for the fact that it vibrates constantly and emits a very dull “humming” noise. It really doesn’t bother me, but I’m curious what causes the vibration? Is there an internal fan? Or, is this a sign that the drive may be nearing the end of its life.

Thanks!

I’d say it’s either a defect in the drive causing the motor or platters to wobble slightly, or it is not mounted securely causing the whole drive to vibrate against the casing.

Of course it could just be the normal “hum” of a spinning hard drive.