What is that sound?

Bought a brand new manual i30 n line and when i accelerate during the acceleration when the pedel is pressed down i sometimes hear this super satisfying psssst sound from the back of the car ( like air being sucked in our out), i belive that its coming from an exhaust. What exactly makes this sound and why do i only hear it sometimes, not always when accelerating? Because id like to hear it always 😅

(May i add, the sound was alot more present when i bought the car but not after about 3k miles i only hesr it sometimes.)