Nearly silent if mounted on foam. If you screw it straight to the case or even use gel then it will be noisy as will any pump. I mounted mine on sound insulation/pump filter foam and its easily as quiet as my old Eheim 1250.
The Black IceŽ SR1 platform is the new PC radiator technology from Hardware Labs designed for ultra-low airflow applications.
Black IceŽ SR1 radiators feature a custom compressed core width utilizing streamlined 45 micron copper fins giving it unparalleled performance from sub-800rpm to 2000rpm fans and flow constricted systems.