GungHo

I get this strange, loud exhaust "whuffle" (kinda sounds like a big dog scarfing food in my backseat/trunk) from my '06 GT sometimes when I'm at a light or stopped in traffic. It sounds like the exhaust and it only happens at idle, and only happens when I'm on my brakes. It doesn't really sound like it's coming from the wheels, even though I only hear it when I am on the brakes at idle. It's definitely behind me and I can hear it clearly in the cabin. And, no, it's not my dog (no pug fur in the Mustang!).

I haven't had the chance to get out and see if I can hear it from the outside or have it verified from soneone on the outside (though have had people inside say they can hear it). It's not terribly consistent, though when it starts, I can hear it usually when I brake over the next few blocks. Then it will go away for awhile.

Any ideas, or is this normal?