Does it happen if you play silence?
Not sure exactly what you mean by "play silence," but yes, the noise I'm hearing is clearly there when, for example, one song fades out (to the noise) and the next fades in (over the noise). I can hear a split-second break in the noise during the song transition. Note that when I say "fade" I mean the music, as produced; the noise is constant and does not fade.
Sorry I can't find the words to be more descriptive of the noise. It reminds me of the description of some of the sound effects in the later Star Wars movies, where they describe holding an electric razor up to a stainless steel mixing bowl to make the racer engine noises in episode 1.
The noise also seems to interact with the harmonics of certain songs to create a really weird effect. If you can get ahold of the previously mentioned Rush album, listen to "Cygnus X-1" (the last track) at high volume. Between about 1:15 and 2:25 of that song the noise interacts with something (bells?) that has the same effect with me as fingernails on a chalkboard.