I'm also having this problem. In fact, I'm on hold right now with motorola as I call to swap my second phone for the same reason. I started having the problem on my first phone a few weeks ago. The second phone had the buzzing problem as soon as it arrived. In both cases the phones have buzzed intermittently and tapping them face down hard against my palm seems to stop the noise for a bit. That isn't a good option because you have to tell the person on the other end to hold the phone away from their ear while you do it. It suggests to me that there is something like sand in the speaker.
We have three phones. One other has had almost no use, while the third has had a huge amount of use as phone and audio player using the headphone. Yesterday the speaker on that last one stopped working altogether. It only works with headphones. I assume it doesn't recognize the headphones have been removed. I don't believe it had the buzzing problem.
I love the phones otherwise.