That was my first guess too since he mentioned start up and accel, although I'm not sure how a bumpy road would come into play, unless the fuel sloshes and changes the sound characteristics of the pump. If you idle the truck and rev the engine, does it squeal? If so, have someone rev it for you while you climb under there and put your ear to places like the gas tank possibly

 

Nope, only when accelerating. Can rev all i want and it won't make the sound. We think its a bed bolt, because you can bang on the passengerside bedrail and it will squeak.

 

Ahh, I guess that it has something to do with the way the bed flexes when you accel then...that explains the squeak over bumps too...

 

yea sounds like you got a bolt or two that is a little loose. or even a suspension bushing????? can you feel a little bit of vibration if you shake the truck on the passenger side? if so its just a bed bolt. if not then id lean to the suspension

 

Haven't tried rocking the body. I hit the bed rail and followed the noise. Sounds like its under the bed rail towards the tailgate????

 

Is the noise still there with the tailgate open? Any chance that it's coming from the leaf spring to shackle connection?

 

Hmmm haven't tried that yet.