I have been having the same problem and my iphone 4s is only 3 days old.
I recently has the same loud buzzing with my iPhone 4S. I setup an appointment at the genious bar and when I got there the girl looked at me like I was crazy. I was told that it is normal and that i shouldnt worrry about it. I love apple but she did not seem to care. i told her some of the demos did not make that noise( i tested prior to my appointment). She even told me hers did it, and let me hear the buzzing from her. I didn't hear anything when I listened to her phone.
She then told me that her phone is new but eventually will get loud, when I asked her when she purchased her pho e, she hesitated at first and then lied to my face that it wasnt even a day old.
I insisted that she replace it since I have only had it a couple days but She said that there was nothing that apple can do about it as it the vibrator. I actually believed her when she said that but never imagined that others would have gone through the samething. I can't believe I was lied to just so that they can get through their appointments ASAP.
Moral of the story guys is this, dnt believe what they without doing research first and Whatever they say is because they are just trained to do a certain job a certain way. No offense to them, she was just doing his job and didn't look into it further than that. But because of this specific thread and all of you having the same issue I will not rest until i get a proper functioning phone that i paid for. So whatever they say, just insist they replace it. I hope this story helps some of you.