when you hit a bump it sounds like a dull rattle any ideas ???

 

on my s-crew they had to reposition some pads that help isolate the bed from the frame...worth a look?

 

I had the same thing, it sounds like there is something rolling around in the box. Check along the passenger side frame rail, there is a long rectangular piece of rubber that is supposed to run the length of the frame rail, on mine it slipped out and was hanging down. Take a big screwdriver and lift the box enough to tuck it back in.