There's not a lot of space left in a 1U server, and to get the right airflow, the airspeed has to be much much higher as what you'd get in a normal ATX case.
Can you replace it with quieter small fans, while maintaining good airflow? Sure sometimes. Is it ever going to be "quiet"? Not a chance.
Of course 1U servers are the most extreme case. Having rackmount equipment at home is fine if you can put it out of the way and pay a bit of attention to what you buy - i.e. a 3U or 4U machine will usually have much more normal fans you can swap to quiet ones. Some people also swap fans on 1U switches.
Contrary to what some sibling comments suggest, I am currently using an older HPE-2530-48G-PoEP switch with the 3 builtin fans swapped for Noctua nf-a4x20-pwm (installation went very smoothly). They are actually less audible than the disks of my NAS next to it. Admittedly, I am only using about third to half the ports and the load is only moderate most of the times but this setup has been working flawlessly for over a year now.
I guess it depends on your workload and the specific hardware involved but using server hardware with quiet fans in a living room environment can certainly be achieved.
Server fans are like 12V 1A, Noctua don’t make that kind of fans.
Fan diameter actually corresponds to server thickness. 1U use 4cm, 3U and 4U use 8 or 12cm. Power consumption stays the same, which means monsters like 1U Xeons come with like horizontal 8-unit array of a pair of contra-rotating 4cm 12V/0.5A(12V x 0.5A x 2 in series = 12W per unit) fans.
The fan will be more expensive than the one it is cooling and it wont make much difference in sound and it might even overheat because the silent fan run slower less air flow.
Aren't Noctua fans quite large, like 12cm in diameter? In a 1U chassis, your fan can only be around 4cm in diameter, because they have to be mounted at the back. You can't mount them horizontally, because of course rack units are meant to be stacked close together.
Not sure how relevant my case is for the discussion because I have a dual socket Xeon workstation in a tower case. I built my rig with the big noctua coolers with brown fans. They are super quiet and highly recommend them.
There aren't any silent server fans. They're small high speed ones (they have to be because of the height restriction) so there's no way you can put a large low speed fan in like you can with a desktop. Plus servers run hotter precisely because they can have loud fans with lots of airflow.
There's not a lot of space left in a 1U server, and to get the right airflow, the airspeed has to be much much higher as what you'd get in a normal ATX case.
Can you replace it with quieter small fans, while maintaining good airflow? Sure sometimes. Is it ever going to be "quiet"? Not a chance.