What is this part?

I'd like to know what the round black thing in this picture is:

http://guide-images.ifixit.net/igi/nVAuB...

I'm referring to the thing located just above the aluminum tape highlighted in the yellow box.

In my MacBook, it is making an annoying buzzing noise similar to the noise made by that old playing card in the bicycle spokes trick. It has a feel like a speaker cone and when I press down on it the noise mostly stops, although I can still feel a vibration. That makes me think the noise is originating elsewhere and is simply being amplified by whatever this thing is .... I suspected the fan at first, but when I stopped the fan (by wedging in the tip of a screwdriver!) the vibrations continued, and when I removed my finger from the black thingy, the loud noise returned, even with the fan stopped ......

Any ideas?