The timescale is on top in seconds. So the pitch sharply jumps every couple seconds. But also this seems to vary and can be more rapid. This was very audible for me and made the device useless. I also wonder why others haven't figured this problem right away. When you once hear it with some static tone then you cannot unheard it even with more complex music.

There has been talk of weird audible effects, group delay measurements and uneven frequency responses with some methods. These are all reflections of this same underlying problem. Maybe some buggy frequency locked loop generating the DAC reference clock.