that's a ground loop issue. there are many ways to skin the proverbial cat... galvanic isolation is the ideal solution, albeit not a free one. if you aren't already, try running all your gear through the same outlet. I know that may sound like a hazard, but it's highly unlikely you'll exceed 15amps of power, and it will minimize the chances of a ground loop.
another point of interest - we have a pair of yamaha hs80 monitors, and these things are very prone to creating a ground loop problem. believe it or not, the power cord needs to be plugged in just right on the back of them to ensure proper grounding takes effect... so sometimes a little jiggling is just what the doctor ordered.
and yet another point of interest - your audio interface and your computer. usb powered interfaces are prone to pick up ground loops. we hacked a usb cable to ensure power didn't go through it from the computer. then we connected it to a little usb cable that comes coupled with a plug for a dc type transformer and used a 5v transformer with it to re-establish power into the usb cable. finally we connected this to the interface. it's a hack for sure, but it got rid of the ground loop!
unfortunately, ground loops are a huge pita due to the difficulty pinpointing the culprit in the audio path, but there is light at the end of the tunnel. hope this helps