Mistracking-what instrument or type of sound...

is most likely to produce it?

Thanks much, I am and will be evaluating some cartridges.
The Hi Fi News test record has several tracking "torture" tracks (side 1, tracks 6-9). Typically used to set anti-skate, they provide a 300 Hz test tone, and each track increases the amplitude. In other words, each track is successively harder to track without introducing distortion in the form of a very audible buzz.
Keep reducing the tracking force on the cartridge and you will hear it. Once you do, you will know it forever.
Very well recorded classical piano, provided it's "big" piano music like Beethoven or Liszt. Speakers Corner reissue of Clifford Curzon playing Liszt is my toughest piano LP. Ashkenazy/Beethoven sonatas on English pressed Decca/London is not quite as tough, but close.

Very well recorded operatic tenors and baritones. Operas on German EMI are the toughest-to-track LP's I own.