I dare say this is nothing new but thought I'd post it as it may help those looking too build quiet rigs etc.
Being a stickler for noise, in particular the intermittent type that vibrates through the whole chassis at random intervals, e.g. mechanical hard drives and in particular 7200rpm drives. I pinpointed the cause of most my case noise coming from the drives being secured to the chassis (On rubber mounts) with screws etc and the vibration being transferred through the case. My OS drive is an SSD, but I also have a 7200 for downloads and everyday use, and a couple of Greens for media storage.
I tried a couple of different things and cage setups, but to no avail. Then I remembered how they used to suspend turntables from steel cables or chains in clubs with serious sound systems to isolate the stylus / arm from the vibration of the bass.
So I recreated this in the case, suspending the drives so they touch nothing, other than four anchor points, thus isolating their vibration from the chassis. I can tell you it works 100% I can't hear them spin up or spinning and it's cut down noise dramatically. No more pulsing or whirring nonsense.
There's probably lots of materials you could use but after having to move my intake to the front of the chassis, the webbing I used initially impeded air flow considerably, so I settled on wire traces / leaders that you normally use for lure fishing.
I secured them to the chassis using 4 screws above, using a couple of M4 nylon washers on each screw to clamp the traces between. The case is Anidees AI6, obviously your anchor points will vary from case to case but this should give you a rough idea of it's done.
I had to cut out the soundproofing around the front and back drives, so they didn't touch anything and give room for cabling etc. Made absolutely no difference noise wise and the gains from the drives being suspended far exceeded any losses in that area.
My case only has:
x1 140mm Noctua intake
x1 120mm exhaust (Pulling through a Kraken X31 on CPU)
RM850 that's pretty much passive
This is easily sufficient to power an overclocked 980ti, 4790k etc at good temps, and on days like this the room temp in the top of the house must be near 30c. Only reason I'm adding any spec is the machines very quiet but doesn't lack the horses, to me framerate is is just as important as noise levels and I believe I've finally, after allot of tinkering, struck a decent balance here.
Also, if you're wondering what the black tapes about, that's the 7200, it's suspended within the suspended cage. I can probably get rid of that now and bolt it on to the cage, remnants of my previous efforts (Which is also very effective but very fiddly)