If you have the OEM exhaust, it might be a leak. It could also be a bad spark plug or plugs.
Can you tell if the sound is coming from the rear where the exhaust tips are?
Maybe you can have someone stand outside of the car and listen to where the sound is coming from. If it's near the front or middle I would guess it's a leak.
If it's coming out of the exhaust, I would say maybe fouled plugs.
Just my opinion.