"Strange" noise on 1998 Jimmy

Recently, my wifes '98 GMC Jimmy has started to make a "chattering" noise after putting it in "D" and taking off. It also makes this noise
periodically when backing up as well. It almost sounds like a metal-on-metal sound, and doesn't bog or jerk when driving. Also, after driving a bit, the noise seems to subside. But after driving a bit and coming to a stop, like at a light/stop sign, and then taking off again, the noise comes back, particularly while RPM's are up in before shifting (auto trans, 4.7L V-8, 150,000 miles). I checked the drive shaft and it doesn't appear that the U-joints are loose/bad. The drive shaft to the transfer case seems to turn freely when in 2H ; Trans fluid is still red and at proper level. Also, I get the noise whether in 2H or 4H. Lastly, the noise is not there when in "P" with engine running/revving, so I'm assuming it's a drivetrain problem, but not sure where to start looking.
Any help would be greatly appreciated. Don't want to have to go to the dealer if possible. I pray it's not a transmission problem, or if it is, a small one.
Thanks for any replies in advance. Andy

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