I just took a quick look at mine. It appears that the speaker just screws to the bottom of the sub and there's a grill over it. The screws that hold it on are visible. Try to tighten them down (gently) and see if the buzz goes away. If it doesn't remove the screws, remove the speaker (be careful of any wires that come are attached to it-don't stress them and note their color/position), and inspect it. Blow some compressed air (gently) around it to see if you can dislodge anything. Try using it outside of the sub enclosure if the wires are long enough. Turn it all the way up and see if it does it and if you can determine where on the speaker it's coming from. If it only does it when mounted in the sub, try to narrow down where it's coming from and see if you can use some foam/cardboard/tape/etc to insulate it in that area.
Hope that helps.