Most likely cause would be a collapsed engine mount letting the two pieces of the metal bracket hit each other.

it does not do this all the time, it makes the jerk when I am going a declination. and i have noticed that by the time it shift down it make the jerk and the sound. I though transmission is the culprit but you are expert and now better so how can i confirm if this is the engine mount?

Have to look at them to see if the rubber isolators are collapsed, torn, squished out, etc. Sometimes you can see them move when a helper runs the engine and shifts between reverse and drive.

OK, I checked it thoroughly with the help of a friend, and I found no such movement, no signs of smashing, all engine mounts looks good. I have noticed that the jerk is only when it shifts from 1st to second while the vehicle is going down a slope and if i am not pressing the gas pedal. with pressing the accelerator pedal (slightly) it shifts without a jerk but with a slight jump. and if i push the gas pedal (full) and keep it pressed it accelerates and shifts smooth without jerk or jump, but you can always do that, specially in traffic queues.

What you're describing sounds like "bump shift". Mostly that occurs when the transmission engages one of the clutch packs with more force than is needed. That is felt when there is not enough load on it which is typically at low speeds. The dealer can reflash the computer's software with updated shift schedules to address about ten common complaints. I did that on my '93 Dynasty when it was new, but it didn't solve that. It's just the nature of the beast.

Since yours is also making a metallic sound, don't overlook worn universal joints. Look around the bearing cups for signs of reddish-colored rust.