I took a 737 on a hop from KATL to JFK today and listen to this...
I plugged in FL330 and clear weather with no wind. Fuel was 40% in each wing and 50% in the center tank.
At about FL180, the aircraft started a small, but noticible "rocking" motion. I rechecked the panel to see if fuel was accidentally transferring from tank to tank, but it was not. Rate of climb was 2,500 fpm and as I climbed, the rocking became more severe. By the time I reached FL330 the rocking was almost 10 degrees from side to side!
Any idea what that was all about? Anyone ever experienced something similar?