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.