Buzzing noise with USB 3.0

[fuzzymath10]

I recently installed a USB 3.0 card in my desktop (it needed a molex connector). Works fine but whenever I attach a 2.5" hdd, I get a soft but high-pitched whine from inside the case. Switching back to USB 2.0 ports, the noise disappears. I'm curious if this has anything to do with the power reqs of the drive, since I don't need to use a two-plug adapter with the super-speed ports as I do with the onboard USB 2.0. I don't think it happens with things like usb keys.