I have flown on them several times. If you were lucky enough to get one of seats in the front of the plane it was an extremely quiet flight.
On one flight from LAX to Hong Kong the plane hit clear air turbulence while they were serving a meal. The cart almost hit the ceiling and a flight attendant was injured.
Was it during winter by any chance ? The air over the North Pacific can get extremely turbulent during the winter months, specifically about a couple of hours east of Japan.