it sounds like it is too tight a fit...a couple of ideas to try firstly sprinkle talcum powder between the ball and socket as that is what is recommended for squeaky wooden floor boards.
As a last resort after trying everything else suggested by the wonderful people on this board try lightly sanding the ball[b][u] with very fine sandpaper and i mean lightly and keep testing until the squeak is no more..hope you find the solution..