I too have sensed a rapid yaw oscillation - twenty years ago, in a RJ L15, during an early AM reduced thrust TO from DXB. It only lasted 10 sec. or so, and was a high frequency (~2 hz). The crew felt it was engine caused, which I doubted.
But they invited me to observe the gages on the next TO which was at rated thrust. No gage indication, and no sensed yaw oscillation this time.
But there may be some relationship to the 777 situation of rgsaero.