I've yet to find a definitive answer to this, but Google shows hundreds of people asking questions.
I have an M-Audio M-Track, a $100 USB-based audio interface.
Across all 3 computers I have tried it on, there is a high-pitched whine around 1kHz, similar to the one here: https://dl.dropboxusercontent.com/u/7345145/noise_2.wav
That high-pitched noise is in common between many brands of audio interfaces, as you can hear recordings on Google.
This happens when recording with things plugged in and with nothing plugged in with all possible configurations of the box (phantom power, monitoring, gain, line level, etc.)
It's not a ground loop. It happens on my mac pro when it is running from battery or when plugged in, on a 2006 original Mac Pro, and on my AMD-based desktop, always exactly the same noise, same pitch. I've tried putting a powered USB hub between it and the computer and that didn't help.
High-pitched whining is a common problem across many USB devices but there don't seem to be any answers other than "ground loop." It isn't a ground loop.