Dale,
I had a rattle like that in our '04 Monterey for a couple of years and had a hard time figuring it out. At one point I got a friend to go for a short ride in the cargo bay to try to find it!
Turned out Ken at BCS, without even looking at it, said he knew what it was. There were some rods in the front axle system that needed washers/shims at the attachment points. The threads on the attachment studs were short. It took an hour of labor and $10 worth of washers to fix it. I don't know which rods they were or what they were called.
Bill