Thanks all for your responses. Well, I guess I've got a few things to check out eh! I'll have the ball joints checked and the cab mounts and fender bolts to eliminate them from suspects.
The sound is of something heavy rattling, so I'm pretty sure it's not something that fell into the ducts or a wrench that a GM mechanic left in there 34 years ago(but you never know!).
I'm kind of hoping that the original owner had stuffed a tin of cash up into the dash to hide it from his wife, then died without ever telling anybody about it, but I don't think that's it either!
Thanks Again All,
Rick
71400LB