Damn! I live in Romania and here, RB/RZ cameras are pretty rare so I'm pretty sure local repair shop guys never saw one open so I am not sure I feel very comfortable having them open this baby up. Also, the camera works absolutely perfect, I am sure this thing doesn't affect it's functionality as mostly I am using it for portraits and some landscapes so pretty much never with the bellows extended. Also with the 110 attached this thing is barely noticeable and also when handholding the camera it's a lot less obvious than when mounted on a tripod. I don't know honestly ... it may be just a tolerance thing.
LATER EDIT: so I always considered myself to be a little handy man and always got to repair my own things around the house ( completely torn apart my iphone 3G, replaced some bits and put everything back together again ) so I took a shot and opened up the sucker on the bottom part where I knew most of the mechanical parts holding the sliding rig. Everything is nice and tidy - nothing is loose, the camera is well looked after and I am pretty sure I am the first guy to ever open her up over here : http://postimage.org/image/8m1dxsxxz/full
It seems that the bottom plate is more of a protective piece to keep us from accidentally touch the bellows when handholding the camera and less a structural piece to hold the whole sliding thing together nice and tight. Seems that most of the weight of the mount part which slides is held down by the 2 dented lateral plates that get "operated" when we circle the focusing knobs. I suppose that's where I will find my problem although I am really tired now to open the side plates and have a look. I will do that tomorrow although I am a bit skeptical thinking I will find anything loose, I think it's all a worn out thing - to be honest the whole mount part along with the lens attached ( especially the 180 ) seem WAY TO HEAVY and I really wonder how do your RZs don't wobble considering they are held in place mostly by the 2 lateral dented plates.
Although I usually go by following Murphy's law : IF IT AIN'T BROKEN, DON'T FIX IT - I find it a bit annoying especially that I know other RZs don't do this so it's like an itch in my brain ) Anybody has any ideas ?
For the sake of argument - could you guys have the camera mounted on a tripod to test for this - bellows fully extended and gently push up on the lens from under it. As I said - when handholding it - it's really difficult to see it happening.