2011 Tacoma back panel rattle.

AFTER I INSTALLED MY MAGNAFLOW MUFFLER, MY BACK PANEL BEHIND MY SEATS IN MY ACCESS CAB RATTLES A LOT AND IS SUPER ANNOYING WHEN IM IN SECOND/THIRD GEAR WHEN MY MUFFLER GETS LOUDER.
I TRIED SOME SPRAY FOAM IN SPOTS BUT NO LUCK.
IT ALMOST SOUNDS LIKE A BOLT LOOSE, BUT THERE ARE NONE. I PULLED THE PANEL AND SAT IN THE BACK WITH MY WIFE DRIVING AND I CAN PINPOINT IT, BUT IT'S ALL ONE BIG SHEET OF METAL BACK THERE.
I TRIED A PIECE OF WOOD BETWEEN MY BED AND MY CAB TO COMPRESS IT A LITTLE, BUT NO LUCK.
ANY HELP?

 

Try pulling the plastic off and putting a towel between the plastic and sheet metal

 

If that doesn't work, then just rip all the plastic out. The vibration vibration from the exhaust won't go away