The fact that it varies from disc to disc suggests you're seeing film grain or noise inherent to the mastering of the disc - all discs, DVD or Blu-ray vary in quality.
The reason why you don't see it with composite/S-vid is that the resolution is lower, and thus the deficiencies of the original disc are masked.
Consulting Editor, What Hi-Fi? Sound and Vision / whathifi.com
Audio Editor, Gramophone