I have been experimenting a bit with mine and the noise issue. I have found you can hear a rustling sound quite distinctly while playing with one of my amps that has a very high bandwidth - 10mhz.
This is different to the intermittent issue I had previously which was literally like a radio station coming out of the speakers.
My suspicion is its to do with the lack of a filter on the output of the DAC and the very high bandwidth of this particular amp.
Thanks
Bill