if the dealer had the car for 2 weeks and couldnt figure it out ... what makes you think we can do it from thousands of miles away?(i guess you're not in the US)
find another dealer to take it to or post a video of the issue so we can at least attempt to make an educated guess.
where is the sound coming from(front, rear, middle of the car)? is it speed dependant
pr RPM dependant? does it do it in every gear? does it feel like anything is hitting anything? is it coming from inside the engine? did you try to duplicate it while standing still? does it do it more if you apply heavy or light throttle?
just from your vague description i would suggest checking the flex disc, that is the rubber disc connecting the transmission to the driveshaft.
let us know.