TOSLINK out to external DAC - high end noise

[scroono]

Hi there - I have had the Ultra for a few weeks now, set up and working well with toslink cable into a schiit multibit dac. Everything had been great up until a few days ago when a high end noise started appearing (tied to the track, for example if there is lots of cymbals this would occur a lot). Hadn't been there before; when I remove the toslink cable and just use the WiiM internal DAC, the high end futz is absent. Has anyone had similar problems?