The other day, before descending, I took walk to the rear toilet and did notice that in cruise the bus moves around quite a bit. The air was still, but the tail was moving around ever so slightly as opposed to a very stable feeling in the cockpit.
I also asked a hostie who was sitting in the back, what she thought of the approach and she mentioned that the bus does tend to be quite "unstable" in the back. This confirmed what you have observed.
But as I said, the tail has to move to make the whole A/C move. The longer, the more leverage (i.e. A321)