Jack up both rear wheels and support the axle tubes with heavy duty jack stands. Start the engine, put the transmission in gear, and listen for the noise. Then carefully inspect all the moving parts to see if you can locate the noise. Stay well away from the moving wheels if you choose to crawl around under the truck.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!