I'm there with you guys. I have had the two speakers in the passenger door replaced under warranty to solve the same issue.
To diagnose and repeat the problem, they have a cd with tones that sweep from low to high frequency. Regardless of replacing the speakers, the sympathetic buzzing vibrations at about 125-250hz (low vocal frequency) prevails.
It'll go back to the dealer where they'll most likely find you're correct about that pocket material, and stick on a piece of duct tape, and charge someone $100.