I noticed the same thing years ago on smooth night flights. A coupled roll/yawing, very smooth and hardly noticable. Took it to be a very sudued dutch roll (like the autopilot had almost conquered the oscillation but a bit remained). Perhaps someone can explain. Nothing to worry about eg like climbing out of Washington in heavy turbulence, being thumped against the 747 cabin wall sitting next to a nun who continuously prayed and counted her rosary beads (aren't these people supposed to have faith ? - I do wish they wouldn't do that)
