Does your external HD have a fan? If so, the fan might be dirty or unbalanced, which can contribute to the noise.
If the noise is coming from the HDD's themselves, then you might be able to put rubber washers between the HD's and the mounts (take out the small screws holding the HD's in, then reinstall them with a thin rubber washer over the screw before you screw the HD's in).
Michael
If the noise is coming from the HDD's themselves, then you might be able to put rubber washers between the HD's and the mounts (take out the small screws holding the HD's in, then reinstall them with a thin rubber washer over the screw before you screw the HD's in).
Michael