I had what sounds like the same problem. I ended up finding a CD that played a bunch of the right frequency and would cause the scratchy, buzzing noise at loud volumes. It had been happening only a once or twice during a movie if at all.

It ended up that the top midrange driver on my M22 needed to be tightened down a bit more because after I got my allen wrench out and tightened it up a bit, it went away.

Hope this helps.