At first I thought it was pax changing seats, but it's not... I start barking at them every time I hear a seat belt click open. This movement isn't nearly as pronounced as pax seat hopping. Could be some fatty shifting about though lol.
Never the same pax either, and I only ever notice the odd movement with pax on. So guessing it's either tied into a heavy load which made me worry about a crap tranny mount, or I'm stressing about nothing and you're right about someone shifting around picking out a wedgie.
Could it be fuel sloshing about in the forward tank under the middle seat row? Can't remember if that one has barriers to stop fuel from sloshing.