Every now and then, I run into sibilance issues while playing LPs. It doesn't happen often, but it does tend to pop up once or twice during the regular listening week, prompting me to start thinking about what could be causing it.

For the most part, my cartridge/tonearm appears to be set properly, since I'm getting perfectly natural reproduction of the consonant 's', as it is being enunciated by the vocalists. But on a few occasions, it does get distorted. And it never fails to startle me. So my question is: given that there could be several reasons for this distortion, what, in your experience, is the most frequent cause? Is it the bad source (i.e. badly recorded vocals), or is it the worn LP, or is it the bad groove tracking? Or is it merely a not thoroughly cleaned vinyl?