This is a known issue and is DEFINITELY the PEV (Passive Exhaust Valve). I don't have the PI number available but tell your dealer to check the TechLink bulletin from June 2014. Here is a cut-and-paste from the pertinent part of the TechLink (there are actual pictures in the online article):
The second noise condition is a whistle noise heard coming from the muffler area of the exhaust system. To diagnose this condition, temporarily lock the Passive Exhaust Valve (PEV) in the open position.
Equipped on all light-duty trucks, the Passive Exhaust Valve, located behind the muffler, acts as a broadband tuner in both V4 (Active Fuel Management) and V8 mode for a more refined sound.
Rotate the Passive Exhaust Valve counterclockwise. For testing purposes only, install a 1/4" x 20 x 2” steel bolt, nut, lock washer and two large standard 1/4" x 1” outside diameter washers (body washer) in the Passive Exhaust Valve.
If the condition is reduced or changes greatly in severity with the bolt installed, replace the exhaust system.
The vibration condition may be noticed in the seat or steering wheel when the truck engages V4 mode (Active Fuel Management). After verifying the condition, check the exhaust system for Ground Outs at the mounts and repair as necessary. For testing purposes only, install the bolt, washers and nut as described above. If the condition is reduced or changes greatly in severity, replace the exhaust system.
My dealer and I have been monitoring mine for a few months. As said above right now the only fix is to install new exhaust from the muffler back and the same style valve is still installed so many begin to squeak again in a few months.