Remember this one for the future as its with most cars.
If it sounds like a rex/rotary, it means one or more of the cylinders isn't firing (unless its a rex/rotary lol).
Easy way to check what cylinder it is, while the car is running, unplug coilpack number one and see if the note changes. If it does it aint that one. Plug it back in and continue to do all of them until you find one that doesn't change the exhaust note. That is the cylinder the car isn't firing on. Could be as simple as a loose plug connection on the coil pack or a faulty coil pack or plug.
Pretty easy really