I have seen countless threads from people asking why certain model of HDD makes so much humming, vibrating noise, while others claiming it is totally silent. The culprit lies in manufacturer variance.
HDD being mechanical device, obeys standard manufacturing variance rules. Even for the same make / model HDD coming from the same assembly line, some will be balanced better than others, results in totally different idle acoustic attributes.
I have used dozens of HDDs, below are some of my personal observations:
I have bought 4 WD6400AAKS drives, manufactured around similar date, one is totally silent, one makes slight vibration, and two makes absurd vibration which makes the whole case resonate.
I have bought a ST2000DM001 2-platter drive to upgrade the silent 640GB for my workstation. This drive was shacked (harvested) from a Buffalo external 2TB drive and is somewhat silent, moderate amount of vibration, dampened with rubber mount and is good for workstation use. Due to this experience, I recommended this model / series of HDD to my dad, when he bought a ST1000DM003 (single platter), it vibrates so much the case turns to a humming noise making machine. I have to resort to all sorts of suspension to make it tolerable, very bad experience.
I bought a WD10EARS green drive which has the same amount of vibration as my ST 2TB, even though it is running only at 5400RPM and has same 2-platter design. Several years later I upgraded it to a WD50EZRX green drive, with its 5-platter fat-ness it runs totally silent, no vibration, no humming.
I got a WD My Cloud 1TB from my friend, with a WD10EFRX red drive which is totally silent and vibration free. But his My Cloud 2TB with a WD20EFRX vibrates a lot more. The WD20EFRX with 2-platters and 5400RPM gives more vibration than my WD10EARS green, with the same casing, RPM and number of platters. The "3D Active Balance Plus" technology in the red drive apparently cannot coupe with the badly balanced platters.
All those can be seen as anecdotal evidences, but from my perspective, buying the same drive does not always gives you the same acoustics is tried and true for me. Manufacturing variance is real and manufacturers do not always try their best to balance the platters in the drive, if you encountered a mad vibrating one, it's just bad luck. If you have high standards regards to noise of your computer, try to buy several HDD of the model you intend to buy and pick the best behaving one and sell the rest. This may sound like a lot of hassle and effort but that's the price to pay for a totally silent system.
Discussions welcome, I'd like to learn from other's experience regards to this topic.