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