General solution;
Throw a small person inside your trunk,...and keep him there until he can figure out this strange sound...
Don't forget to feed him once in a while!
That actually is a good idea, I used to do that alot, but only with trusted people...
Bring a Snickers bar in case....
If you can get the car up on a lift, just start grabbing things and shaking. Try to duplicate the sound with the windows up and down. If you hear it the same way with the windows up, it's in the car. If you hear it better with the windows down, it's outside the car.
Also, place your hand or someone elses hand in the area where the sound is coming from to see if they can feel any vibration/bump against the floor to isolate if it's a tranny mount. If it's a tranny mount, I wonder if it would make the same sound going from drive to reverse? My brtoher's old Trans Am had a bad tranny mount that only made noise when doing about 25-30mph around corners, a lound banging under the console.