One of the big voodoo things Dell does is the hard drive rails. There's a strip of semi-soft rubber between the rail and the drive, and the screws are mounted in rubber feet, so nothing touches the hard disk except rubber. It's amazing how much vibration thos buggers can cause. And vibration is the root of all sound in your computer. Example, take a loud fan and hold it in your hand, while it's on. No matter how loud it was in your case, they're all quiet when they aren't making your case vibrate.