Thanks to everyone for the input. Please excuse my ignorance but this is my first 2-wheeler since my teenage years (and that was so long ago I can barely remember)
Ok, I checked everything mentioned so far and the only suspected source of the "rattle" sound is the chain. I noticed that the chain does contact the white plastic (maybe nylon or delrin) guide/guard at the bottom of the rear sprocket. When the bike is on the side stand or held upright (no stand) under its own weight, I can grab the chain ahead of this guard at the point where you would measure chain freeplay and vigoroulsy move the chain up and down. The chain makes a "slapping" sound when it hits the inside top and bottom of the guard. I can imagine that when the rear tire is bouncing over the rough ground this may be the "rattle" that I am hearing. When my 225 lb buddy sits on the bike and the swing arm moves up, the chain tightens enough that I can't create the "slapping" noise.
However, I don't think the chain is too loose. The sticker on the black plastic chain guard above the rear sprocket calls for 40mm (1-5/8") of chain freeplay and the chain is currently adjusted to this spec. That is when the bike is on the side stand our upright under its own weight. As mentioned above the chain gets tighter as the swingarm travels upward under more weight. It does not look as if the chain can be tightened enough to prevent the "slapping" against the guide and not be over tight when the swing arm moves up.........Or am I wrong?