buzz in the back

I have had an annoying buzz/vibration coming from the back of my '07 GT for approximately 6 months. Drivers side near the backseat and trunk area. It was never really a problem because I could just do the turn up the radio technique to make it go away. haha. I finally took it to my local dealership (Greenway Ford) and, after a week now, they have diagnosed the problem as a manufacturers defect regarding the weld of the body to the frame.
I have read on this forum that these cars have some inherent quirks and rattles in them but this seems a bit excessive

Has anyone out there experienced these types of vibration problems and what was done to resolve the issue? I can't wait to get my baby back baby back...this rental they gave me is the worst!

Here is the latest update : They still can't find my buzz. After two weeks now the car has been gutted and rear glass removed. They have gone over every weld point and admit that the sound has gotten worse. Now, an engineer from Ford has been summoned with engineering diagrams to further investigate this head scratcher.
WOW is all I can say. At least they let me get rid of the sucky Mazda 3 rental and "upgraded" to a V6 mustang. I will keep you posted with the results.