Thanks for all the input.
Mounting it may add pressure to the case so that the structure or the thin top starts to amplify vibrations coming from the drive cage. As said, loosen the screws a little if they are tight. Another thing that could possibly change the noise is changing the position of the drives in the case, as there are many places to choose from.
I think this might help. I chose this case because how I can have relative good airflow in the main chamber. I am quite hesitant to use the top chamber for drives, but, that being said, the top hard drive cage is rock solid while the bottom cages have quite some flex (I can bend the sides with my hand).
It's referred to sometimes as "beating", and is occurs when the hard drive's RPMs vary slightly (they don't exactly have the same RPM), and serve to either amplify each others vibration, or lessen it. I tried soft mounting, loosening screws, grommets, etc, and the ONLY solution that worked for me was suspension. I know it's not the answer you want to hear, as it takes considerably more work, but at least you know of someone else who's gone though it and beat it.
I currently run all of my storage as NAS in another room for this very reason. Some people don't notice the "beating" at all, but it was ultra super annoying to me.
Hmm, thanks, I'll look into it. I had the same hard drives in my old P180, and a Corsair 550D setup I tested but abandon. The vibration in both cases were heavy enough to transmit through the case (as it does in my fractal). However, the fractal takes it to a whole different level, the "beating" is as loud as a cat at times (it comes and goes). I am considering setting up my hard drives in a separate DAS case now... This is very frustrating... I can only imagine what you went through.
I live in a small studio in NYC, so no option for separate room placement.