There are a couple more possibilities that come to mind. Spark plugs are one, I'm not sure if the change interval is close to the mileage on your car though, need to check an owner's manual for that!
Could also be carbon buildup causing that too, does it make a tappy sound at all? (The engine itself, I mean)
Another possibility is intake; do you hear any hissing if you pop the hood while the engine is running? Particularly around the intake hose, but probably not the hose itself.