Update, so now both speakers are doing it.
I took it in to the dealer yesterday and was frustrated to say the least. After a lengthy back and forth about whether there is or is not a problem, I replicated the problem with the tech and service manager using 65hz and 80hz tone tests. I got them to document the issue. They are trying to talk me out of repairs by saying opening the door panel could potentially cause more problems, and that the issue doesn't seem to be so bad. Their proposed fix is a two day service to install insulation/dampening foam. I'm done with this dealer on account of how long it took them (10 hours!) to call me and tell me nothing is wrong. I'm going to try my local dealer to see if they can do the repairs before attempting a tear down of the door on my own.
For those who are unable to reproduce the problem at the dealer - I can tell you that my speakers need to "warm up" before the rattling/buzzing starts. playing some bass heavy music or the tone test for ~5 minutes will ensure the buzzing will be replicated.
I try not to editorialize, but damn, I'm already fed up with all of the minor problems. Stereo, preconditioning, and trim have all been problematic. I really regret buying this car.