Vibration at high rpms in low gears...

Ok, whenever I go to shift from 1st to 2nd or second to 3rd, i get this vibration noise in the back of the car....maybe when the trnny is....all i know is that its behind me...this happened on my 03 gt and i just ignored it but now its bugging me...I took it to a mechanic and he said the car runs perfect and theres nothing wrong...to me it sounds like a heat shield or thin piece of metal vibrating....Anyone have a clue what im talking about?

I checked the spare tire....I even took it out and went for a ride...still vibrating! Its so frigin annoying.I hear my exhaust roar and can almost predict when ill hear the noise. It does it just as i need to shift. Its kind of like a pain in the ass shift light except its a noise!