Add me too, I think.
Whine from the rear passenger side at around 55 mph and over. I took a mechanic from the local Ford stealership and it happened during acceleration or keeping the speed at the same level. They said it was happening at 'tip', because when I took the foot off the gas, the whine went away. The claimed it was normal on Mustangs.
Now, read this from the March 2013 edition of Motor Trend, page 76.
".........we liked the Mustang quite a bit on the road, despite a rather noisy whine under load on the freeway."
This was a 2013 GT manual, with 3.73 gears.