For the longest time I've thought my muffler hanger was a little messed up and allowing the muffler to vibrate against the rear bumper.

WRONG.

I finally get under the car to see if baning on the muffler will move it back into place. Then I notice the hood somehow is vibrating, like a rubber bushing is gone or some kind of support is weak on one side. It's only loose enough to vibrate at idle.

 

try tightening the bolts to the trink locking mechanism...well if thats what you mean by hood?