One simple thing you can do to reduce rattles, particularly in things like divider and butt-bar hinges and pins is to put them under tension. Just get an assortment of heavy-duty bungie cords and be creative. The object is to "load" the structural parts such that the loose joints are held firmly in contact and so less likely to vibrate/rattle in sympathy with the trailer motion.
Rattles in places like the suspension, floor, and walls are much more difficult to address, but sometimes if you can get some of the very viscus grease designed for heavy equipment into the gaps it will quiet things down for awhile, then you will have to re-apply. Try using a very large syringe with a correspondingly large needle to dispense the grease into otherwise inaccessible areas.
Alas, assuming the trailer design was good to begin with, the rattles are an indication of wear in areas of metal to metal/metal to wood contact, and other than replacing the worn components (often difficult and prohibitively expensive), there just isn't much you can do. Save your money for a new(er) trailer with fewer rattles.
Steve Jernigan KG0MB
University of Colorado