ok, it all started when i was pulling an empty 14" heavy duty flat bead trailer a couple of weeks ago. i was making a left hand turn crossing two lanes. I went over a couple of good rolling bumps right after i accelerated from a stop. I hear this grinding or scraping noise from the back of the truck. It sounded like I dragged the trailer jack across the asphalt. So, i stopped as soon as i got across the road to check it out. I found nothing. so i assumed i just dragged bottom. As I accelerated from the stop i heard it again. I unhitched the trailer and it went away when i accelerated. I drove a few miles down the road and hit a bump and there it was again. I crawled under the truck and couldn't SEE anything that looked broken or out of place on the rear axle or drivetrain. So over the past few weeks i have determined the following: It happens when i am moving and hit a bump. I stood on the rear bumper and used all 250 lbs. of me to bounce the truck and can't seem to make the noise, it only seems to happen when i am driving. It was really bad that first day and the days following, every little bump, even just pulling into the driveway, made the noise. Now it has gotten less sensitive. It still happens but isn't as bad and usually only happens when i am moving faster and hit the bumps harder. Understand i am not talking about hitting bumps hard enough to bounce my passengers around, it is just normal road driving. Driving over speed bumps at 25 will cause it too.
Any suggestions? I am stumped and would rather get a good idea for what could be wrong before i take it to a shop.

 

The noise could be from the drive shaft u-joints.

 

I was thinking the same thing. Crawl under the car, grab the drive shaft and try to move it. Work the front then try the rear and see if there is any play in the u-joints. It may be minimal, so take a very close look as you push and pull.