thumping sound under C4500 Duramax

[etwaddle]

I have a new C4500. Dash was noisey from vibrations. Loud thumping sound under truck on certain road bumps. Thought it might be the fuel tank hitting the frame but can't prove that it is. Does anyone else have this sound?

[lineman]

First of all welcome to the DP!I drive a 2003 5500 daily.I put about 100 miles a day on this truck.I too have the vibrating plastic dash issue, but I know exactly where to hit the dash when it starts....straight in front of the steering wheel, just slightly to the left.Takes care of it every time!It took a lot longer to find my thumping under the truck.I finally realized it usually happened when I turned left,drove on a street that had a slight lean to the right or just drove down a bumpy road.My truck had a keeper pin missing from the battery pan that is located under the top step of the passenger side of the truck.It was on the left side of the battery box.With this pin out,the box does not move much, but makes a hard jolt to the outside.It doesn't seem to make any noise going back to the proper position,just when it makes it's short swing out.After replacing the pin the problem was solved.Now if I could just find a cure for the conastoga( Is this spelled right? )wagon ride.You know, the side to side, top heavy swaying while going down the road feeling.This has been the hardest truck for me to ever get used to driving.The steering is way,way too touchy.I don't belive a Corvete is this touchy!And hit any uneven pavement,you better hang on to your a-- because it is just plain scary.I now have about 4000 miles on it.No Dmax or Allison problems at all.But check the battery box.I hope this helps you out.....Lineman

[Heartbeat Hauler]

Did you look underneath to see if you haven't inadvertantly backed over a small import and the owner is trying to alert you :D
Also, possibly a weak exhaust hanger allowing the system to bang on the frame or rear suspension. Sound will travel along the frame and body parts.
JP