When I quickly turn my 13 inch late 2013 rMBP I feel something heavy moves a little bit, Could it be the battery?
It doesn't sound good. On my 2011 everything is packed in pretty tightly so the parts probably wouldn't move around much even if they were all unscrewed. The "retina" machines are even smaller so I can't imagine that there should be anything moving around in there either...
I don't have any plans to take it to apple, It works like a swiss watch. They might then glue the battery with their strongest glues
True, but something loose means it banging around potentially causing damage. Its glued for a reason.
You need to take it in!!! Those lithium polymer cells are very fragile and if it were to come completely unglued (which it might do), the wires connecting the cells could rip and short and cause a fire. Seriously. Take it in. Sorry Mike Lol didn't mean to quote you...