I'm after some easyish cheap ways of making my s40 engine sound nice. I've hurd people mentioning the air filter before, but they look round in shape on websites and i know from bits and bobs of servicing in my car it looks like a rectangle filter pad. Does this require a bit more modding? What else will help with sound? New exhaust maybe? I also hear filters can help fuel economy?

Any ideas or suggestions would be great!!