Thank you for the response.
I'm not desperate to build one myself, but my understanding is that prebuilt NASs are rarely very quiet. A self-build using the latest components should achieve quieter solution that is also more energy-efficient.
If you build it there is a big chance it can be lowered to where its inaudible, but at the same time if this is your intention, then i would go a case like antec solo II that has suspension for 2x 3.5 hdds, but this will also make them not hot swapable, so something will have to give.
Personally i still think Synology is a good bet for your situation, i had a DS111 and personally i didnt feel it was too noisy, and the stations are so small that you can place them almost anywhere, you only need a network conection and power, then it can be placed anywhere. Now my experience was decent not saying silent but i also didnt have it on my desk, was below a rack where abient noise was higher. Some of the later models have started using 120mm fans (mine was like 60mm if i remeber correctly, but mine is also a 1 disk so not much space for a fan, 2x hdd sometimes use 80mm)
Good luck with your choice,