Mine were never loud at all. That was of course before I broke up the array to use them as single drives. Even now they aren't loud. Rubber washers might help, I guess...assuming vibration it the cause of noise.
if it is the 'squeal', that is usually internal to the drive. Isolation mounts aren't going to do you much good in that case. The little rubber mounts do help for all the other noise the drives make, though.