I'm having a similar problem with my M30s. I would occasionally notice a buzzing sound out of my right speaker. I tightened down the wire connectors and also tightened the screws, but the sound is still there. While listening to a Peter Gabriel CD, there was a particular song that kept causing the buzziness. The sound was most noticeable from the back, coming out of the port. I eventually found that as I was turning the speaker, to hear where the sound was coming from, it would actually quiet down and even go away as I would turn the speaker over. Tapping on the sides of the enclosure seemed to have quieted it down, but it is still there when upright. It definitely sounds like there is something in the enclosure that is vibrating at certain frequencies. I have both speakers upside down now and that seems to have temporarily fixed the problem.
Other than the occasional vibration sound, the speakers do sound good, especially with high quality recordings with a nice dynamic range. I hate to have to go through the trouble of returning them, but if the sound continues after I put them upright, I'll have no choice.
