I've had a lot of time in the 206L over the last 2 years and have noticed this on a few occassions. However, i have always thot it was a cramped pax, in the back pulling him/herself up on the seat to try and get a better seating position. It gets cramped in back when 5 pax in there. Unless theyre all midgets of course or 5 supermodels, you get the picture.
People moving around is my guess, just like Helonorth describes.
Cant say ive ever felt the fuel sloshing around enuf to bump the aircraft forward or aft?
HJ