It may be some decoupling issue.
I had some problem in the past with that with various stuff: Xonar essence One, Trends Audio TA 10.2P and even Rotel RB-1552.
Point is: ground loops or low quality (switching) power supply are poison for the whole chain (RB-1552 was hissy and noisy).
DAC (even good ones) suffer from input voltage fluctuations (and ground loops): noise comes from signal and power inputs.
For the most part I was able to reduce noise simply insulating ground (that is: I duct taped the ground on the plug!) and/or use TOSLink.
None of this is easily feasable for Pi2 so you can use an USB decoupler between Pi2 and your dac or move to Schiit-like stuff (I'm waiting for the infamous pile): modi has a galvanic decoupler.