What level of noise is indicative of a failing disk drive? I recently added a 750GB WD drive to my Dell XPS410, and I'm not sure if the noise is coming from it or from the original 500GB system drive. The noise is not constant - sometimes I don't notice it for a few hours. But then it comes back. It is sort of a periodic or pulsating whirring sound with about a 2-second period, lasting for several minutes.