Driveline noise

[Motorhead]

My truck is making a small klinking noise when I put it in gear. The noise is coming from the driveline somewhere. My truck is a short bed with the one piece aluminum drive shaft. I had a VERY trusted friend put the trans into drive then reverse with the brakes set while I was under the truck looking for something loose. I didn't notice any slop in the U joints. Sometimes you can put it in gear and not hear the noise until you take off. The noise is only heard once in each direction. No vibration is felt going down the road. It's like a short cracking noise. I just turned 20,000mi. Any ideas to tell the Dreaded Dealer? Chris