buy purples, they 5400rpm.. you wont hear em unless u use it as a pillow
The fans in the NVR's are usually louder than any hard-drives. If my NVr was driving me nuts I would just move it.
I'd figure that you'd hear more noise the clicking of the read/write heads than you would of the actual platters spinning. Isn't continuous recording going to wear out the circuitry of SSDs a bit faster than normal usage? I haven't kept up on SSDs too much, and I remember they have a limited read and write lifetime, that they were working on improving.
this is one reason why I suggest external PoE; so you can put the NVR some place away from your wiring point.. you can hide the NVR some place it wont be heard, or most importantly.. found.
you need to allow air to circulate, at least through natural convection.. meaning some place up high in the void for warm air to escape, and some place down low in the void for cool air to replace it.
if you just seal electronics off in a closed space, you'll get thermal runaway.. my LPR camera was hitting 150 degrees inside with IR off; then I drilled a few holes and remove the PoE splitter so it had less heat and more airspace.. now I never see more than 105F in the middle of the summer w/IR on.