I have this problem on the passenger side. I took apart the door and found that it was actually the plastic sheet that covers some of the door innards. I actually found that I couldn't prevent it from buzzing even if I used applied all 10 fingers to various parts of the sheet. I think that if any part of the sheet is under tension (any two points on its perimeter), it will act as a drum.

One solution might be to peel the sheet off its gooey mount (not easy--maybe impossible--I tried) and reapply it w/o tension. The other option might be to completely cover it with a sheet of Dynamat insulation. I didn't have any so I used a sheet of foam which may have helped.

I more or less decided to live with it for now. I think I only have 2-3 songs that have enough bass to get it buzzing so it doesn't happen very often.