I suggest that you keep after the dealership until they figure it out and "fix" it.
Like said above, most likely some part of the exhaust system it hitting another part of the car.
Back in 1970 I bought a brand new MGB-GT. Something around the transmission rattled under acceleration, so, I took it back to the dealer. They "fixed" it.
I asked them what was rattling, they said the transmission case was rattling against the transmission tunnel. I ask them how they fixed it. They said they took a crowbar and pried the transmission tunnel further away from the transmission case.
Sometimes the "Bubba" method of fixing things works best ......
humfrz