Damn I tapped things with a rubber mallet and it went away and now I can't make it make the rattle again lol.
When I rotate the crank by hand back and forth I don't hear anything weird from the chain area.
Edit: Pinpointed vibration issue to the warped v-band on my downpipe. Easy fix, next week. Who knows what the other rattle is but I know that this noise is solely from the v-band being warped beyond belief. The air gap is causing some crazy harmonics as the exhaust squeezes out.
Still, curious what this was. Can't reproduce it anymore, just was a random observation that one time.