That's your sound tube. Ford pipes some intake noise into the cabin through this to make the quiet muffler sound a little more aggressive. You can safely pull and cap it if you don't like it.
REALLY?? Sounds nuts but I guess it's better than LOUD PIPES outside!!
Don't think I paid any attention to interior sound since I immediately went to Magnaflows!!
I removed mine. The result is a much cleaner looking engine bay. The interior is noticeably quieter. All I hear now is the exhaust. I have an Airaid CAI which can be installed with or without the sound tube, and includes the necessary plugs if you chose to remove it/ Here is a write up from another website on how to remove it: http://themustangsource.com/forums/f800/sound-tube-delete-parts-list-503349/
I know the car in the write up is 2010 with a 4.6 but the tube is the same.
Here is a kit, with the necessary plugs. This makes it simple but I think you could get the plugs cheaper on your own: http://store.racingintegration.com/mustang/11-13-mustang-gt/intake/sound-tube-delete-kit.html
Thanks for the reply Svtpilot!!
I plan to remove mine today and found an expandable rubber freeze plug that fits the intake tube hole perfectly and a Master cylinder gasket for the firewall!!
I'll let ya know how it works out!!
I took the gray foam out of mine today and it sounded terrible so I just removed the whole thing and it sounds much better to me with my Magnaflows!!
I found a good way to plug the intake tube for less than $5.00......an expandable freeze plug from Advance Auto Parts.....works GREAT and looks important!!
I used the Master Cylinder Gasket from Dorman (#42098) on the firewall......looks OEM!!
Now.......on to the next project!!