i have 2 thoughts.. when i first got my truck i had a a fuel rail come loose and truck was covered in diesel, all the way to the hitch. after the dealer cleaned it up (truck was stock at the time) it would clunk whenever i would go over bumps or potholes, etc. they said it was the leaf springs. they lubed them and it lasted about a week. they couldn't figure out what it was so they replaced the leaf springs and my problem has been fine.. and i've also seen many trucks missing the round plastic bushings on the lowest leaf spring..