Mine does this as well, 2006 GT Automatic. It almost sounds like the tags on my dog's collar jingling. Mine does it in a wider RPM range, pretty much up to 2500+. I can't hear it once it gets beyond that point over the exhaust and normal engine noise, but I think it is still there.
The noise seems more pronounced on acceleration and at constant RPMs. I usually notice it the most while cruising on the freeway between 65-70mph (~2200 RPM). It is considerably more quiet during deceleration.
I honestly don't believe it is pinging. I've heard my car ping, and this noise is different. I even at one point ran my 87 tune on 91 fuel with about 10 degrees of timing taken out to test if it was pinging...the car ran like CRAP, didn't ping, but still made this other noise.
I've inspected the exhaust hangers in the TSB mentioned, and mine are solid. I've also looked at the exhaust heat shields and the plate between the motor and transmission. Everything seems to be tight and normal as far as I can tell.
My car has done this since I got it at ~23,000 miles (at 46,000 now =\). It hasn't gotten any louder, although I noticed that after going to 3.73 gears I hear it more. (higher cruising RPMs)
The 2nd gear on my transmission whines something awful as well...I had Ford tear it down while it was under warranty (about 12,000 miles ago) and they said nothing was wrong with it. The car has continued to make both the whining and this tinny noise without causing any problems so far.
I've pretty much written it off as a "normal" sound. I keep telling myself I'm going to go find another 2006 Mustang with an automatic at a dealership and test drive it to see if it makes this noise, but haven't gotten around to it.
I am glad to hear that my car isn't the only one making this noise, but I really wish I could figure out what it is!