FINALLY - SOLVED!! It's the 120V AC plug where the center console meets the center dash. I removed the center console trim piece and could shake it and hear something rattling in it, the trim piece is just held in place with the snap-in tabs, here are pics of the bottom of it and the end of it where the power outlets are. The 120V outlet has 2 plugs that go into the back of it. The plugs come out by using a small screwdriver to press in the tab on the side of the plug. Then remove the outlet from the trim piece by pushing in the 4 straight plastic tabs on the sides of the outlet. Once it's out, remove the back by prying up the 4 U-shaped plastic tabs. The back assembly includes a little circuit board with copper receptacles. Trapped inside each of the 2 receptacle is a white plastic piece with a spring behind it. The copper is not tight enough to keep the white pieces from banging around. I just used needle nose pliers to squeeze the tabs together tighter until they stopped being loose, but not so tight that the white piece didn't move freely. While the trim piece was out, I put duct tape in a few places just in case, but I'm not sure it was necessary. Put it all together and now my truck feels solid as a tank again. You're welcome for the free root cause analysis and corrective action, GM. My invoice is in the mail. (I also took pics below of what the center dash looks like naked, sometimes that's helpful for other things so here you go.)
mikepol2's post in 2015 interior rattle deep in dash was marked as the answer