Ok so today I isolated where the rattling is coming from. Seems to be the top front. Whenever I sit on the top, rattling is reduced but not eliminated. So instead of drilling in screws in an attempt to hold the mystery panel down, I cut off the entire side panel that has the access hole. Thing is, everything looks good internally. All areas have generous amounts of glue and feel totally sturdy, so im not sure what is causing the rattling. My thoughts are that it's panel #8 (to those who are familiar with this build). Any ideas?