You know that settling sound some furniture and homes make, right? That "creak" or "pop"?
Well, my PS3 makes these noises when it's on. I could be watching a movie, or game, and here it make a small "pop". It's not an electrical sound, but a mechanical sound as if some plastic piece really is settling. It's not the HD either, took the HD out and put it into my computer and ran a disk check, passes and no noises from it.