Virtually silent. We have a 12-core and you can only hear it if you put your ear close to it. But for an external hard drive enclosure we are using an OWC “Thunderbay”, and that is more audible. Not terribly noisy, but audible. The problem is that the maximum thunderbolt cable length is 36", so we can’t really separate them very much. I am going to see if I can replace the fan in the Thunderbay, but it’s part of the power supply I think, so I might not be able to.