Hopeful that I've found the answer...
An answer!! At least for now. I spent many hours on this site, and on ClubB5, studying posts and suggestions about various dashboard squeaks, rattles, rumbles, etc. It's sort of unnerving to hear how many folks are having variattions on this problem. A summary of what people have discovered by themselves or from their dealers includes: dash retainer, the screw holding the glove box in place, airbag canister either lose or rattling against something (usually the gove box), the "dash support" (behind the right access panel) missing its padding, and the dash rattling against the windshield. All the solutions involve tightening the clip or screw or replaceing the padding between two parts, except one which involved shoving fabric into the space between the dash and windshield (an odd solution in my mind). Today the dealer told my culprit was none of those, but was the HEATER BOX RATTLING AGAINST THE FIREWALL. I think he said the clips are less than ideal and had to be padded or braced w/ felt. Anyway, for the time being, I'm really happy -- the pronounced rattle at 1300-1400 RPM is gone, but the car just sounds better all around. I really hope it lasts. :wink: