I need to pick 3-4 drives for a Unifi UNAS and another 3 for a Unifi NVR. I need drives that will be quiet but still perform well.
For the NVR - I selected WD Purple Pro 14TB drives with a 20/29 dB rating (idle/seek). These are 7200 RPM drives.
For the NAS - I selected WD Red Plus 14TB drives with a 20/29 dB rating (idle/seek). These are only also 7200 RPM drives.
I’d appreciate any feedback / experience before I spend just shy of $2k! 
Thanks!
I have never really taken any measurements of the drive noise made when installed in a NAS.
Very difficult to gauge really, and highly subjective.
dB’s are also a horrible metric unless all samples are tested in the same environment under the same conditions and such. Changing the distance of the mic by just a few inches can impact the measured noise level. A jet engine at full throttle can be only 20dB’s if measured from far enough away… And there are no testing standards that OEM’s have to follow, so take whatever the packaging says with a grain of salt.
Best bet would be to just pour over as many user reviews as possible and try to fish out some take-aways.
Another option would be to find one reputable review site, and stick with just the one (you can do more, just don’t cross compare.). Once again, dB’s are meaningless without context, but a good review site will at least a have consistent test setup.