A little unsolicited advice, but try putting some small gaskets between the screw and harddrive, it eliminates a lot of noise.
My new one has the little rubber gaskets on the mounting screws, but the drive is mounted against a mounting sleeve (?) sort of thing. I pull a little adapter out of the HDD cage, mount the drive into that (with the rubber gaskets) and then put the adapter back into the cage. I think the noise is the adapter on cage rattle.
It doesnt happen that often (so i think it is only 1 drive doing it), so I havent bothered to look into it. But with moving parts youll always have SOME noise.