Same issue here. I just noticed it on my 16gb black 4s. I haven't noticed it until today at work. I made several test calls from my desk phone and they all buzzed. My wife's phone call earlier in the day is what made me notice it.
This is really annoying, had mine for 6 months now and it's a horrible high pitched buzzing noise every time I call since I had it, makes me want to end the call every time. It's a great phone for everything other than why it exists in the first place, telephone calls.
Now you may be wondering why I still have it after 6 months in that state. I live in Belgium and bought it from my ISP and they have third party to deliver the Iphones and I called them and they said they would have to send it away for minimum one month!! This I find outrageous. So will have to buy the 5 and then I can send it for repairs. Not good Apple.