a friend of mine recently purchased a Mark V complete. when she peddles or spins the wheel while is it off the ground, there is a continuous buzzing sound coming from the drive train. all parts are stock and the chain is new and properly lubed. chainring and crank bolts are tight and the pedals are tight and greased. the chainring and cog are clean as well.

the sound is similar to a healthy drivetrain when it's lifted up and spun hard but much louder. the sound seems to be coming from the rear wheel more than anything, although it's difficult to pinpoint exactly where it comes from. I took the chain off and spun the cranks - no sound.

any ideas of what could be causing this sound and how it could be reduced?