I studied music theory and composition for about 10 years, so I'm intimately familiar with something called the devil's triad, the tritone, or the "devil in music". It's a diminished 5th, such as the interval between C and G-flat. It has a dissonant sound, although the original sound was quite different than what we hear now (due to the way tunings in instruments have changed). It is best describe, however, as "restless". Our ears "want" it to be resolved into a consonance. Modern ears don't hear it quite the same way as people did hundreds of years ago, because the interval is used quite commonly now. There are such things as diminished chords that are very common, and they don't necessarily convey as restless a sound or as dissonant a sound as they once did. I can think of one song that repeats several dissonant chords as the basis for the song, and some of the lyrics are, "people get happy whistling a melody". Obviously, this is not a demonic or even a sad song. Anyway, I couldn't find a formal writeup of the phenomenon, but I thought some of you might be interested in this portion of the wikipedia entry on it. It says it was considered the devil in music from the 18th century, but if I recall my music history training, it was considered the devil's triad well before that (if I recall correctly, at least as far back as the 13th century, if not earlier). If you spend any time studying the progression of music from about the year 1,000 AD, you'll see that while music has gotten increasingly dissonant, our ears have adjusted to it so much that early music is so plain as to be almost unlistenable except by those who cultivate a taste for it, and modern music with all its dissonance sounds downright consonant to most people.