I have heard some real disturbing noises when the truck gets flexed when off-road. Afterwards I get out and check the body to bed lines to see if anything is out of wack. All the braces and rivits and welds look good under the truck, I think that it might be the bed flexing while under stress. I could be wrong but everything else looks good and it only happens when I am in a ruff enough spot to lift a tire off of the ground. I guess axle attriculation is not the strong point of a H. D. truck and something has to flex. Just my opinion, Thomas