I have an 05 coupe and made a custom sub box for the trunk. I spent some time tracking down the internal buzzing in the cabin and am pretty happy with the results. But the problem I'm having is external buzzing at higher volume. Of course, the first place I checked was the license plate. Fixed the problem there. But there's more. The buzzing seems to be coming from some place behind the rear bumper cover and above the muffler. I've stuck my hand up behind there and grabbed everything that I could reach to see if I could find the part that is buzzing...to no effect. Any ideas?

 

Try checking in your spare tire area...could be a loose bolt or just something might have fallen in under the mat.....happened to me many time before.