I am not SURE about this... but I've had my rear bumper off to do a "factory" hitch install and noticed there are vents with "one way" diaphram covers for air on either side of the FJ at the rear corners. I am assuming this is exhaust from the interior. I guess this could be a spot where air could generate noise...
I will be posting my hitch install soon and you will be able to see pics of this then - a couple of hours from now hopefully.
PS - Title of post says "car" - *BAD* FJ owner!

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2007 Toyota FJ Cruiser - Titanium Metallic (2JR) - 4WD MT CQ 3T EL 5A 6D CF
OEM Upgrades: 17" Alloys / Tow Hitch / A-Trac Switch / High Pitch Horn / Locking Lug Nuts
Other: Window Tint (@ 32%) - LT285/70R17 Goodyear Wrangler SilentArmor Tires
GeorgiaFJCruiser.com