Lightning bed ((crunch/pop)) when driving

I have a 2003 Lightning w/ 35k I have had it for about three month now. I am so annoyed by the crunching and popping noise coming from the bed when i turn corners and pull into my drive way. The guy the dealership said it normal on all the lightning because of the fiberglass beds.... i took out my bed liner and it still does it. So i figure i would ask my boys to see what i can do to fix this problem? Thanks