I've read several reports of people who feel a buzzing or vibrat-ey sensation when they are suffering from Mirena symptoms. I have also seen a couple of people mention muscle twitches. This and my own personal experience has let me to a hypothesis.
I was laying on the couch yesterday, having another lovely day where I felt like doing nothing (these have come much more often lately, unfortunately, but I'm in my 4th month (when many people crash), and detoxing from oxalate overload, so I guess I shouldn't be too surprised), when the twitching started. Earlier in the day, I had had the buzzing feeling, and it got so annoying I finally laid down until it went away. Having both of these fairly close together, though, got me to thinking...what if the buzzing is just a more internal manifestation of the muscle twitching? In effect, what if it's just muscles deeper inside my torso rapidly tensing and releasing that's causing the buzzing feeling, and I don't identify them as muscle twitches because they're muscles I've never felt twitch before?
Also, for what it's worth, it seems my buzzing and twitching happen more often when I'm a little low on fluids. I don't have to be massively dehydrated, only a little, for this to occur. Might not be the case for everyone, but it's worth a shot to guzzle some water the next time you get the twitchy/buzzies, and see if it goes away in 15 minutes or so. I just wish I'd figured this out sooner!
I also suspect this may have something to do with low magnesium, but since I've had some issues getting magnesium into my system over the last few months, I have yet to determine optimal magnesium dosing for myself, so I can't say whether or not that helps (yet).