01bluz28

I have a 2001 camaro z28 with the 6 speed. Last summer while driving the car it had a very annoying rattle every once and awhile. It almost sounded like a heat sheild rattle or something but i could not find any loose members under the car. It does it when i move in reverse and times and when the car is shifted and works struggles to gain speed. If i just drive it hard and gas it all the time there is no rattle, its just when im taking it easy and crusiing around town. Could this be something in the trans??? someone told me a throw ring??? Let me know if anyone has an idea of what could be wrong.