I was having this annoying issue with my RME Hammerfall II, which had a constant coil while noise on my speakers, even though I was using balanced cables.
I suspected that the problem was that I was powering my RME Hammerfall II with firewire.
I just got a power supply, to power the Hammerfall, and the noise is completely gone!

Also the popping noise when turning on and off my computer and waking up from sleep is gone.

Thought it might be useful to someone with same problem.