Flaps do vibrate quite horribly sometimes. The 737 ones do with small flap extensions when speedbrakes are used. You know the designers and builders are well aware of it, but I admit, it is very disconcerting to see. On the VC10, we had a periscope that went up through the rear fuselage roof so you could see the engines and tail. It was a surreal experience watching the whole tail fin shaking backwards and forwards when on training flights we did Dutch rolls (I don't know where the Holland connection came in!). These are slow yaw/roll reversals of large scale. The whole fin would be moving. Not my favorite exercise!