Noises coming from the rear or trunk area? It might be this...
 

There are (2) ventilation doors that allow air to escape the cabin when the trunk or doors are closed or when the fan, heater or AC are on.

These vent doors are located underneath both sides of the rear bumper behind the rear wheel wells.

Even with the fan off, if fresh air is entering the cabin through the ventilation system or windows... these vent doors are doing their thing.

While driving you may hear them gently open and close... that's what they are designed to do.

It does allow some outside road noise to enter the cabin, especially with an aftermarket exhaust.

They are accessible from inside the trunk or when the rear bumper is removed.

Most cars have them. Video clip a few posts down...

https://i.postimg.cc/ZY8vfbDp/001.png

https://i.postimg.cc/cCCq84v2/002.png

It would be so great if you're right about this. That noise has been bugging me for year and I haven't been able to figure out what it is. I've searched all over under and around and each time I look I get more annoyed that I can't figure it out. I'm gonna owe you a beer ;)

Although the principal is the same the actual parts on the Twins are very different from the picture shown. For one it is on the other side of the car and it is much much smaller. The flaps are just a small piece of rubber not a rigid slat that would bounce and rattle. If you are getting a rattle it isn't the vent.

https://www.google.ca/search?q=su003...AHTcG4EtYmg-KM

Was worth a shot.

In action on a WRX with an aftermarket exhaust system.

[ame]https://www.youtube.com/watch?v=sDvvB8CHjGc[/ame]

Updated

i've had some really kicking sound systems, and while i have heard those flaps slightly fluttering outside the car when the sub is pounding, there's absolutely no way to hear it inside the cabin.

https://static.nhtsa.gov/odi/tsbs/20...40470-9999.pdf

Here is a TSB with the revised part numbers for these. 72651CA001 for the BRZ

They can get stuck open in some cases.

My 2016 FR-S had a highway-speed (ie vibration in the chassis) ticking noise. It was worse in colder weather and seemed to go away when the windows were down.

Dealer tore the whole interior apart trying to find the source. They concluded it was this vent. They replaced it but the ticking never went away. This was likely before this redesigned part was available.

Mine didn't sound like the video above, which is more of a shutter sound. Mine was more of a ticking sound. Like raindrops falls on tin metal.

check your trunk dude..










https://25.media.tumblr.com/d820b236...qijko1_500.gif