I tried a lot of things, it was a persistent thing. At first I had the DAC plugged into the hub on the cooler, so I though, oh, easy fix, plug into the laptop header, but there was absolutely no change in the pitter patter of fan static... So I tried adjusting the fan speed on the cooler while listening and that changed it, and turning it off changed it, but with it off there was still a low hum. Only unplugging it removed the noise completely.
Yeah, I would have thought those things would cure it too........... strange. What brand cooler are you using?
How about a different port on the laptop? I have 6 ports on my laptop. the two on the side are connected to the same header as two on the back, but there are also 2 ports on the back that have their own separate headers. I usually connected my usb transport to one of the separate headers and everything has always been quiet.
I have an external cooler too, but I power it separately from an old 5V cell phone charger.