My since past 97 1500 5.7L Suburban's dash would rattle at highway speed when in overdrive while towing. This was due to maxing out the towing capacity at 130K miles in overdrive. Not smart to do with that vehicle. Its a chevy and it lived to see another day. I doubt its your transmission, but the thought came to mind.
Mine rattles at the top of the drivers side area too... I narrowed it down to the clip that holds the dash trim bezel on. Grab it near the air vent and pull it back un-hooking it slightly; My rattle goes away..
mine does in the cold as well, dash on the pillar. took off the dash then checked th tightness of everything, found the gauge cluster wasn't seated just as it should be. realigned it(although I probably caused this on the installation of the Sirius SAT radio SR-1) snugged it all in. haven't heard the rattle in sometime.
good luck with your rattle. Drive you nuts, hated it.
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