I have a "floating" lump about the size of a small marble that occasionally shows up in my left knee to the right side of my kneecap.  I can move it around a bit under the skin using my fingers.  It is very sensitive to the touch and in some positions makes it painful to walk. It is strange that it disappears and then has no negative effect. I can't figure out where a lump that size can disappear to and "hide". My thought is that it should be surgically removed.