My 06 had the almost same problem. Here's what my dealer did to repair it under warranty:
Cause: Exhaust Flap in Rear Muffler is making noise. Found a leak in the vacuum supply hose.
Correction: Replace vacuum hose. Road test, ok now.
After they replaced the vacuum hose, the exhaust drone noise at around 2000rpm near 80km/h(50 mph) is also gone. Hope this info is helpful.