Mike, my 40 does not rattle at all. Basically as the vertical posts tilt backward, the top of the metal wedge on the back of the post is supposed to jam against the black rubber wedge, and once installed and with the latch locked, the posts should be under a good deal of tension and unable to move at all.
Try installing the loader with the wedges up all the way, and then slide them down as far as they can go, against the metal wedge on the posts. Then uninstall the loader (you don't really need to take it off, just unlatch and tilt the posts forward) and move the wedges down a little bit more. I'd guess 1/8" will do, but you may need to experiment. If you've got it adjusted right, you should need to lift the front wheels off the ground to push hard enough against the wedges so you can close the latch.
If your rubber wedges are down all the way already and not jamming, something is definitely not right. I don't see the adjustment of the horizontal supports you mentioned - can you elaborate or take a pic? My supports are held on by two bolts on each side, but I don't see any adjustment there.
Tim