At first I went for a Noctua NF-A6x25 FLX fan, which is 12V powered, and replaced the original fan. I had to cut the connection plug from the original fan and graft it to the Noctua one. Making it just a 2 pin fan. This meant no speed control 
But I thought it would be a better option.
As it turns out it was too loud and was bugging me. I tried using stand-offs, but this made the situation worse. I think it was vibrating with the case.
While watching the I Built A 4-Bay NAS Using A Raspberry Pi 5 I noticed the “thin dust filter” mentioned. 
→ the solution was to use PVC PC dust mesh filters instead of the original case covers. As it turns out there are plenty of options on Amazon!
The new covers also had magnetic edges, but the screws were not enough, so I had to get more metal on the case. While cutting the PVC dust filters using a cutter, I thought … wait a minute, these are metal segments that I can glue to the case! 18mm on the left, 9mm for the right.
Now I have a quiet NAS, the fan only kicks in when I do something with the NAS. The led lights make it fancy looking too.
