Depeding on a few things like windy day,dry weather, etc. I would hear a faint twang type vibration from the rear outside of the car. Speeds between 50-60 mph. Some days no noise. While cleaning beneath the rear facia are under the bumper lip I accidently duplicated this sound as my sponge bounced off the facia about 1 ft. to the left or right of the muffler area.
Turns out the vibration sound was from the heat shield cover above the muffler. Under certain stress forces caused by air passing by there while underway this shield sounds off.
Simple fix if you find your shield has a noticeable sound while tapping on the facia just left or right of your muffler. Use a wooden paint stir stick,usually no cost from any store selling paint products, approx. 1 ft. long & 1/8 inch in thickness. Gently run it from the back of the heat shield between its own lip & the end of the facia just below it. It will fit snugly in place with about 1 inch of the stick showing after you push it towards the rear bumper until it can't go any further.
Drill a 1/8 inch hole,I used a small hand drill, through the wood & through the lip of the shield. Insert a sheet metal screw snugly into place to prevent any movement of the stick from then on.
It works real slick. It's just the tension needed on the shield to prevent anymore vibration sound being transferred into the cockpit. If the 1 inch or so of wood that protrudes from the underside of the facia bothers you, just paint to match your undercarriage before installing. It doesn't show unless you lay under the car & look up to locate it, so it shouldn't be any big deal. Sure is worth getting rid of the annoying sound while under way.
Enjoy
Crossfireone