The only nylon-string guitar I've ever owned was about 32 years old when I bought it a few months ago. There are tiny notches under each string, even the unwound ones. Are those there on purpose or just an effect of years of wear on the bone? I would have thought bone could hold up to plain, smooth nylon indefinitely.
The reason I ask is that the 4th (D) string will easily "clink" in an out of its notch with minimal side pressure while the other five all stay firmly settled (when tuned up to pitch). I think that may be contributing to a bit of a harsh tone I get on that string from time to time.