I got a similar noise when plugging the mustang through USB to the PC. In my case the reason is a ground loop (and or ground voltage leakage maybe) between the PC and the mustang.
If you are using a laptop, the easy fix is to unplug the laptop from the power adapter and run on batteries, and they noise is gone.
as time goes by I'm more and more convinced that the only computer worth using for a DAW is a MAC laptop, for this and other reasons.
If this is the reason I'll be SOL for awhile. Although I guess I could go the Pod Farm route. Thanks everyone for your ideas. I'll take a closer look at all of these.