Funny to read this here. I "complained" about this uncomfortable feature of the 777 in another forum, and all the others tried to convince me that I´m crazy and that the 777 flies smooth as silk...
I would support the 2 Hz statement in favour of the around 1 Hz, I think the amplitude of about 5 cm is quite well matching my estimation.
However, I was seated close to C/G, so I would rule out the rudder (or yaw damper) to be the reason, I believe this is an eigenmode (normal mode) of the aircrafts elastic structure involving the engines swinging sideways on their pylon and the rest of the aircraft against it (to keep the C/G steady). It surely is excited by lateral gusts, but I believe the phenomenon does not include areodynamics.
And they don´t do it all, it is especially those with the heavy engines on soft pylons which do it. The difference between A340-200/300, A330 and A340-500/600 is remarkable (with the effect increasing in that order). 747-400 does it. 777-300ER is worst. Never felt it on the smaller aircraft (frequency too high to be excited by lateral gusts) or on the MD-11. Never felt it on aircraft with tail mounted engines. For some smaller aircraft you can clearly feel the yaw damper working (e.g. CRJ 200). On some you can feel worn rudder / rudder actuator bearings, but the frequency is more in the 3-4 Hz range then, and you only feel it in the rear cabin.