Rattle is driving me CRAZY, HELP!

[71400lb]

Okay all...I have a 71 3/4 ton long bed 2wd Cheyenne.
It has A/C, disc brakes front/drum rear.

Now the problem is that there's a rattle coming from what seems to be behind the glove box, but in the lower dash. It's hard for me to describe, but it's a "heavy" rattle, not tinny or squeeky, it's more of a dull and heavy rattle.
Picture a Q-ball in a plastic bucket, and every time you hit a bump, the Q-ball bounces a bit, that's the best way I can describe the noise.

I have taken out the entire glove box, cardboard insert and all, and I cannot see anything loose, or even any exposed bolts for tha matter! I looked under the truck for a loose shock absorber, but it was tight. I drove the truck with the glove box completely out, and it still had the rattle.

Any ideas?

Thanks,
Rick
1971 C/20
400/400