I have a Dahua NVR but I'm sure this would work with anything.
Noctua 5v fan (runs at about 15db at 100%)
Noctua A-Series Cooling Fan Blades with AAO Frame, SSO2 Bearing (NF-A4X10-FLX 5V)
3-pin reducer cable to get the fan running at 60% (even quieter and less than whisper)
Universal 3-pin CPU Speed Reducer PVC Cable Cord Lead for Electric PC Laptop Fan...
Dahua NVR Quiet Silent Fan Mod
Noctua 5v fan (runs at about 15db at 100%)
Noctua A-Series Cooling Fan Blades with AAO Frame, SSO2 Bearing (NF-A4X10-FLX 5V)
3-pin reducer cable to get the fan running at 60% (even quieter and less than whisper)
Universal 3-pin CPU Speed Reducer PVC Cable Cord Lead for Electric PC Laptop Fan...
Dahua NVR Quiet Silent Fan Mod