I've only heard it in 6th gear, but when your coasting in gear and go for gas, it sounds like the whining noise that comes from older rear gears. Its only at, or around, -0.5 BAR
Sounds kind of like what my rear diff did before I had to replace it. New one still makes a little noise now, but it hasn't gotten worse or exploded yet so I assume its alright.
Mods are listed in the sig. Will they still warranty it with the exhaust, or is that one of those things where its all connected way down the line...? From inside the car, it sounds like its below me (tranny), and not in the rear diff. I heard it about a month ago also, so I know its not the front since WJM just reset the preload on the front diff.