It's the Enclosure (Score 2, Interesting)

Well, partly at least. It's no secret that disk drives are sensitive to vibration as this video showed an extreme case. Keep in mind, since disk drives are spinning at 7200-15000RPM, they themselves create vibration that can affect adjacent drives. The drive enclosure can help reduce the problem with use of shock absorbers and vibration dampeners. Most drive enclosures nowadays, for cost reasons, are no more than just sheet metal wrapped around power supplies, fans and drives, which contribute to the problem.