I had the same problem in my setup for a while. Its still there if you listen really hard but the problem lay mostly in grounding and power. Ground everything. Even things your not used to grounding. And do so all at a central point. Also at the same time i was grounding everything nicely i put in my DC-DC PSU. Thats just from my experience though. Im sure in short time there wil be someone in here spouting about ground loop isolators or similar things. Ive never seen that stuff work though. So basically ground it all and ground it well.