I read some of the threads concerning axle whine and I believe I can hear a similar noise in my GT. If I'm cruising in 5th gear at under 2k RPM and let off the gas, I will hear a slight WHOOOOOOOOOOOOOOOO sound that goes away when you get on the throttle.

I'm almost positive that sound is not coming directly from the rear end, but more like in the driveline area. If it were pure axle/rear-end whine, it would generally make the sound as you increased the vehicle speed in all gears. I do, however, hear a lot of clunking noises coming from the rear end if I don't properly rev match my shifts or wait for the drive-by-wire throttle lag to subside.

I have the factory 3.55s

 

Actually, whine on decel/coast (foot off gas) is very often the gears. Gears can whine on accel, decel, or coast depending on how they're set up. I have driven a couple GT's off the lot that had very loud whine on decel between 65-40 mph. My car had the same sound as yours around 65 mph on accel only. Now, after months of attempts, a have VERY slight gear noise on decel between 55-40 mph.