i recently bought my car and did and oil change on it so it runs smooth and clean except for 1 problem.

when i am in 1st or 2nd gear and i am deaccelerating, especially when i am around 2500 rpms or under i hear a loud rattleing sound coming from under my car.
it does not happen if i have the clutch in, but with the clutch engaged in 1st or 2nd i still hear the sound. it doesnt happen in 3rd or 4th or when i am downshifting so i am wondering if i have a problem with my transmission.

i have the car under extended warranty but i am currently at football camp and wont have any time to take it in for about a month. is this serious and what advice can anyone offer me on this. thank you

 

Noises are pretty hard to diagnose from afar. Could be anything such as the wastegate on the turbo, loose honeycomb in the catalytic converter, loose exhaust. Can you feel anything coming through the shifter to suggest that it is in the transmission?