I'm almost certain this is a common problem, but my new Logitech S120s have this persistent buzzing sound whenever I plug my headphones into them.
Incidentally, the same buzzing sound happens if I touch the speaker's jack to my fingertips, except that it's MUCH louder.
Incidentally, the same buzzing sound happens if I touch the speaker's jack to my fingertips, except that it's MUCH louder.