It's a high pitched ringing sound that blends in with the sound (music, dialogue or whatever you're amplifying). It's kind of buried into the background of the sound but you know something is off. Once you've noticed it, it becomes very obvious and annoying. Simply toggle off your Mimby or Bimby (which I own) and after 15-30 seconds toggle it back on and allow it to recycle. Presto, the ringing is gone. It's a very random thing and happens 5-6 times per year (rough estimate). I don't have any explanation for this "issue" but it's a simple fix, like re-booting your computer or phone.
It is recommended that DACs be left on all the time, especially the Multibit DACs because internal temperature is critical in presenting a proper sound. With respect to the Lyr 3 owners with the MB option does that mean you have to leave your unit all the time (and shorten the life your tube)? Maybe because the Lyr 3 and Jotunheim only offers MB to it's USB input, perhaps the USB is always powered? Not sure.