@KC@Gadgetblues
Let’s say that’s weird at least, as a 4U SC846 is actually one of the easier models to make quieter.
I even managed to make a 2U SC826 (Xeon Silver 4114 system) almost silent by replacing the stock fans with Noctuas and adding a 2U Supermicro active CPU heatsink, also modified with a Noctua fan. Temperatures are perfectly fine, both CPU (rarely exceeds 60°C, even after hours of abuse) and hard disks (12 x WD Red 8TB, even after 24 hours of constant read and write never higher than 45°C, 3 NVMe SSDs (the same), also 2 SATA SSDs that admittedly don't have a lot of work to do).
So it
is possible to replace the stock fans with something quieter, either from Supermicro themselves (there are quieter models for their workstation line) or if that's not enough, third party aftermarket fans like the Noctuas,
as long as you plan your system accordingly. Which means: energy efficient hardware (e.g. in my case, a Supermicro X11SPH-NCTF motherboard that has everything 'on board' - no addon cards necessary), energy efficient drives (e.g. WD Reds) and actually, energy efficient software, a factor that is often overlooked. A non-striped array of disks is a lot easier to cool, for example, as only the disks that are accessed actually have to spin. For media server purposes, as in my case, the drawbacks like lower performance (of the array - cache drives help a lot to counter that) are perfectly acceptable. It also helps to use some IPMI software to control the fans and not just use the bios settings for that, as they usually offer more options and a more fine-grained control.
A 4U SC846 actually gives you even more options. 4U CPU coolers are quieter and more powerful, you have a lot more air in the chassis to begin with. You could even mod the chassis and replace the 80mm fans in the front with 120mm ones.
Plus, you're comparing apples and oranges, a rackmount Supermicro enterprise grade chassis with a Synology desktop NAS. The rackmount Synology models are
a lot louder actually. Don't get me wrong, I've been a long time Synology user myself, still have a few Synology NAS in my possession, including a 12 Bay variant similar to the DS3617xs noise wise. Synology offers good software, easy to configure and maintain, but you pay for that convenience with lower flexibility, both hardware and software wise. A truly powerful Synology system costs a fortune, and is still considerably less powerful than a DIY machine, or even a Dell, HP server etc. and the like. Not to mention that you can't really change or upgrade the hardware. The software, while easy to use and very good, still has its limitations, too, e.g. when it comes to virtualisation.
Those are some of the reasons why I actually went the opposite direction, from Synology / QNAP to Supermicro DIY and never looked back.