Regarding Mitsubushi Grandis - Need your help

I just bought a brand new Auto Grandis. When I speed it up gentle, let said started from traffic light or drive in slow traffic jam (stop and go), it will hear the sound like "roomroom... (I think in between auto change gear from 1st gear to 2nd gear, something like that) ". It was happening also when I gentle climb up to multi storey car park. It happens also when I drive gentle in the smooth traffic, when the rmp reach between 40-60, the sound will appear again. Ocos, if I just speed it up faster at highway or smooth traffic, the sound will not appear. Anyone here drives Grandis and meets the same problems as me? Is it normal? Anyone here can help / share please? Thank you :)