If it is road noise you can try undercaoting. Also carpet padding under the ruber floor mat (assuming it came with a ruber floor mat).
Inside steel shelves are solid but things can rattle. If you have them you might try putting somthing on the shelves like thin wood, ruber, carpet. I like wood shelving to keep the rattling down.
Sometimes you have to figure what makes the most noise. What ever is rattling try experimenting. Sometimes it is a little loose and needs another somthing to hold it solid. Other times to parts are rubbing tugether. Try insulating with a piece of rubber (old cut hose, floormat, weatherstriping).
You did not say what type of truck you have.
Tom
