High quality fan, works great as a replacement for the fans in my ReadyNAS NV/NV+ that have failed.
It's not silent, but for $20, it's a great improvement.
It's a great fan, very quiet, and they supply you with everything to make silent air flow.
Used this on a custom 3d printed shroud for a C2750D4I and it works perfectly and is dead silent.
My fan was going out on my PC and this is a quieter fan.
The secret of these fans' low noise is that they run slowly and produce only a fraction of their rated airflow.
I don't have a flow rate tester, but I compared this fan to a Cooler Master of the same size and smaller advertised flow rate (33 vs 38 cfm) mounted to the heat sink of a thermoelectric cooler.
At 65F ambient, the Cooler Master fan held the heat sink to 75F.
This is fine if the cooling requirement is non-critical or you can use a fan that is grossly oversize for the application, but if you actually need the advertised airflow in a constrained space for a real-world application, you might be best served to try another brand.
Check with Noctua for compatibility before purchase.