1996 Camaro RS auto
Problem: rattling vibration noise (from front dashboard area) only when in gear with foot on brake.
After may hours of trying to find the cause of annoying vibration noise, I have found the culprit.
After partial disassembly of the drivers side dashboard, I stuffed a cloth underneath the main horizontal dashboard ledge (plastic part underneath the windshield)
There are some heavy duty velcro fasteners that secure this plastic panel in place. When the fastener nearest the lowest part of the windshield on the drivers side unfastened, the vibration noise ceased.