I have a 3500 dually long bed ext cab. When I drive over really rough places I hear a bad rattle that sounds like it is on both sides in rear. Other times it seems to be only one side. The dealer says it is interaction with the nerf bars and body. The shop that installed the nerf bar brackets that provide the extra support and they got in the back and we drove over some really rough roads and the banging is definitely towards the back of the truck and not the nerf bars. One of the techs seemed to think it is the springs slapping. My 01 2500 hd was continually loosing the pads on the springs and making a similiar noise. The 3500 noise sounds somewhat different but could be spring slap. I can't find anything loose but it sure it irritating. Have any of you 3500 dually owners experienced anything like this?

Tom
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Yes it's the overload spring's hitting the perches.
Had it happen on my 93 dually too.
Not much you can do other than put some weight in the back of the truck by the tailgate so that the spring's make contact with the perches ALL the time, then it won't make so much noise when you hit bumps. Or just take the spring's or perches out if you don't need the full weight carrying capicty.

Make sure your rear shocks are tight. Trust me. That lower bolt can cause quite a rattle over large bumps.