[Corvette95]

Whenever I put another C4 in the stable I completely take the dash apart and wrap every wire and connector with felt / use felt in every contact zone and torgue every screw and put rubber windshield tape on every metal clip, make a world of difference but not for the dash inexperienced. A common rattle on the passenger side of late c4 is the center of the passenger ac vent . Is if exactly flush with the dash pad all the way around or has it warped up? Also door lock rod is a common rattle.
If you have a fact service manual if has a lot of good sound repair section.