Wait guys... before your send him down the tunnel of dealership hell... Bare with me here... When my car was stock there was a HUMM / BUZZ between 60 mph & 67 mph that came from the rear end... It only happened while holding the speed, no acceleration / deceleration, nor while putting the car in neutral at the 60-67 mph speed...

Not to say that this is the problem with his car, but mine turned-out to be some sort of harmonics that vibrated the heat shield above the muffler during very specific speeds... It took me a few weeks to figure it out... Just another opinion...