I had a rattle in my car that was driving me nutty. Coming from the dash or radio or maybe the console, thought I. Nah. Trunk. The lightbulb in my flashlight in the trunk was rattling, but it (the flashlight) was mounted to the parcel shelf and it resonated and echoed and reverberated so that you couldn't source it until I had my GF ride in the back seat while I drove around.
These cars are acoustic nightmares. I also have a nasty rattle that is actually just the passenger seatbelt buckle hitting the pillar sometimes, depending on how the last passenger let the belt retract. I just move the belt and it's fine.
Check for loose change.
Seriously.
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Current inventory:
F22 & G05
Cadillac CT5-V Blackwing w/3 pedals
Past: E24,E28(3),E34,E36,E37,E38(2),E39(4),E46,E89,E92 (obviously),F01,F06,F10,F30,F87,G12,G30(2),G82(2)