I have exactly the same quad and exactly the same problem. If you grab the front end (rack/bumber) and start shaking it pretty rough you will notice that the tubes connecting the front end to the chassis squeal when moved. The only solution I see to this is to insert a rubber bandage around the tubing fitting inside the chassis, furthermore preventing friction between metal. I might be wrong in the location of your rattle, but this had me worried also, however I just left mine alone, considering that a lot of rubbing would need to take place before the metal wears out!