You don't say where the buzzing is coming from or how loud it is. A little more help please. We can assume from the dealers response that it's from the engine but you should confirm this, also does the sound change during hard acceleration?
Did you test drive this unit before you drove it home, if so did it make that noise then, if you didn't ride your bike did you ride a sample model and did it make the same noise? Why not ride a sample unit at the dealer and if it doesn't make the noise point that out to the dealer. If it does make the same noise it may be (an added sound option) you didn't have to pay for/no extra charge.